diff --git a/libcxx/CMakeLists.txt b/libcxx/CMakeLists.txt
index d699135774ee0b..04596fccdfc923 100644
--- a/libcxx/CMakeLists.txt
+++ b/libcxx/CMakeLists.txt
@@ -414,15 +414,16 @@ set(LIBCXX_INSTALL_MODULES_DIR "share/libc++/v1" CACHE STRING
 set(LIBCXX_SHARED_OUTPUT_NAME "c++" CACHE STRING "Output name for the shared libc++ runtime library.")
 set(LIBCXX_STATIC_OUTPUT_NAME "c++" CACHE STRING "Output name for the static libc++ runtime library.")
 
+set(LIBCXX_GENERATED_INCLUDE_DIR "${LIBCXX_BINARY_DIR}/include/c++/v1")
+set(LIBCXX_GENERATED_MODULE_DIR "${LIBCXX_BINARY_DIR}/modules/c++/v1")
+
 if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT APPLE)
   set(LIBCXX_TARGET_SUBDIR ${LLVM_DEFAULT_TARGET_TRIPLE})
   if(LIBCXX_LIBDIR_SUBDIR)
     string(APPEND LIBCXX_TARGET_SUBDIR /${LIBCXX_LIBDIR_SUBDIR})
   endif()
   set(LIBCXX_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR}/${LIBCXX_TARGET_SUBDIR})
-  set(LIBCXX_GENERATED_INCLUDE_DIR "${LLVM_BINARY_DIR}/include/c++/v1")
-  set(LIBCXX_GENERATED_MODULE_DIR "${LLVM_BINARY_DIR}/modules/c++/v1")
-  set(LIBCXX_GENERATED_INCLUDE_TARGET_DIR "${LLVM_BINARY_DIR}/include/${LIBCXX_TARGET_SUBDIR}/c++/v1")
+  set(LIBCXX_GENERATED_INCLUDE_TARGET_DIR "${LIBCXX_BINARY_DIR}/include/${LIBCXX_TARGET_SUBDIR}/c++/v1")
   set(LIBCXX_INSTALL_LIBRARY_DIR lib${LLVM_LIBDIR_SUFFIX}/${LIBCXX_TARGET_SUBDIR} CACHE STRING
       "Path where built libc++ libraries should be installed.")
   set(LIBCXX_INSTALL_INCLUDE_TARGET_DIR "${CMAKE_INSTALL_INCLUDEDIR}/${LIBCXX_TARGET_SUBDIR}/c++/v1" CACHE STRING
@@ -431,12 +432,8 @@ if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT APPLE)
 else()
   if(LLVM_LIBRARY_OUTPUT_INTDIR)
     set(LIBCXX_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR})
-    set(LIBCXX_GENERATED_INCLUDE_DIR "${LLVM_BINARY_DIR}/include/c++/v1")
-    set(LIBCXX_GENERATED_MODULE_DIR "${LLVM_BINARY_DIR}/modules/c++/v1")
   else()
     set(LIBCXX_LIBRARY_DIR ${CMAKE_BINARY_DIR}/lib${LIBCXX_LIBDIR_SUFFIX})
-    set(LIBCXX_GENERATED_INCLUDE_DIR "${CMAKE_BINARY_DIR}/include/c++/v1")
-    set(LIBCXX_GENERATED_MODULE_DIR "${CMAKE_BINARY_DIR}/modules/c++/v1")
   endif()
   set(LIBCXX_GENERATED_INCLUDE_TARGET_DIR "${LIBCXX_GENERATED_INCLUDE_DIR}")
   set(LIBCXX_INSTALL_LIBRARY_DIR lib${LIBCXX_LIBDIR_SUFFIX} CACHE STRING
diff --git a/libcxx/include/CMakeLists.txt b/libcxx/include/CMakeLists.txt
index a4b97b1d7658cc..86bd89dd4d561e 100644
--- a/libcxx/include/CMakeLists.txt
+++ b/libcxx/include/CMakeLists.txt
@@ -1041,17 +1041,8 @@ set(files
 configure_file("__config_site.in" "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__config_site" @ONLY)
 configure_file("${LIBCXX_ASSERTION_HANDLER_FILE}" "${LIBCXX_GENERATED_INCLUDE_DIR}/__assertion_handler" COPYONLY)
 
-set(_all_includes "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__config_site"
-                  "${LIBCXX_GENERATED_INCLUDE_DIR}/__assertion_handler")
-foreach(f ${files})
-  set(src "${CMAKE_CURRENT_SOURCE_DIR}/${f}")
-  set(dst "${LIBCXX_GENERATED_INCLUDE_DIR}/${f}")
-  add_custom_command(OUTPUT ${dst}
-    DEPENDS ${src}
-    COMMAND ${CMAKE_COMMAND} -E copy_if_different ${src} ${dst}
-    COMMENT "Copying CXX header ${f}")
-  list(APPEND _all_includes "${dst}")
-endforeach()
+set(_generated_includes "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__config_site"
+                        "${LIBCXX_GENERATED_INCLUDE_DIR}/__assertion_handler")
 
 # Generate the IWYU mapping. This depends on all header files but it's also considered as an
 # "include" for dependency tracking.
@@ -1060,9 +1051,9 @@ add_custom_command(OUTPUT "${LIBCXX_GENERATED_INCLUDE_DIR}/libcxx.imp"
   DEPENDS "${LIBCXX_SOURCE_DIR}/utils/libcxx/header_information.py"
   COMMENT "Generate the mapping file for include-what-you-use"
 )
-list(APPEND _all_includes "${LIBCXX_GENERATED_INCLUDE_DIR}/libcxx.imp")
+list(APPEND _generated_includes "${LIBCXX_GENERATED_INCLUDE_DIR}/libcxx.imp")
 
-add_custom_target(generate-cxx-headers ALL DEPENDS ${_all_includes})
+add_custom_target(generate-cxx-headers ALL DEPENDS ${_generated_includes})
 
 add_library(cxx-headers INTERFACE)
 target_link_libraries(cxx-headers INTERFACE libcxx-libc-headers libcxx-abi-headers)
@@ -1070,7 +1061,8 @@ add_dependencies(cxx-headers generate-cxx-headers)
 # It's important that the arch directory be included first so that its header files
 # which interpose on the default include dir be included instead of the default ones.
 target_include_directories(cxx-headers INTERFACE ${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}
-                                                 ${LIBCXX_GENERATED_INCLUDE_DIR})
+                                                 ${LIBCXX_GENERATED_INCLUDE_DIR}
+                                                 ${CMAKE_CURRENT_SOURCE_DIR})
 
 if (LIBCXX_INSTALL_HEADERS)
   foreach(file ${files})
diff --git a/libcxx/include/__cxx03/CMakeLists.txt b/libcxx/include/__cxx03/CMakeLists.txt
index b61442da89d4bf..f080bcac44052e 100644
--- a/libcxx/include/__cxx03/CMakeLists.txt
+++ b/libcxx/include/__cxx03/CMakeLists.txt
@@ -1021,17 +1021,8 @@ set(files
 configure_file("__config_site.in" "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__config_site" @ONLY)
 configure_file("${LIBCXX_ASSERTION_HANDLER_FILE}" "${LIBCXX_GENERATED_INCLUDE_DIR}/__assertion_handler" COPYONLY)
 
-set(_all_includes "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__config_site"
-                  "${LIBCXX_GENERATED_INCLUDE_DIR}/__assertion_handler")
-foreach(f ${files})
-  set(src "${CMAKE_CURRENT_SOURCE_DIR}/${f}")
-  set(dst "${LIBCXX_GENERATED_INCLUDE_DIR}/${f}")
-  add_custom_command(OUTPUT ${dst}
-    DEPENDS ${src}
-    COMMAND ${CMAKE_COMMAND} -E copy_if_different ${src} ${dst}
-    COMMENT "Copying CXX header ${f}")
-  list(APPEND _all_includes "${dst}")
-endforeach()
+set(_generated_includes "${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}/__config_site"
+                        "${LIBCXX_GENERATED_INCLUDE_DIR}/__assertion_handler")
 
 # Generate the IWYU mapping. This depends on all header files but it's also considered as an
 # "include" for dependency tracking.
@@ -1040,9 +1031,9 @@ add_custom_command(OUTPUT "${LIBCXX_GENERATED_INCLUDE_DIR}/libcxx.imp"
   DEPENDS "${LIBCXX_SOURCE_DIR}/utils/libcxx/header_information.py"
   COMMENT "Generate the mapping file for include-what-you-use"
 )
-list(APPEND _all_includes "${LIBCXX_GENERATED_INCLUDE_DIR}/libcxx.imp")
+list(APPEND _generated_includes "${LIBCXX_GENERATED_INCLUDE_DIR}/libcxx.imp")
 
-add_custom_target(generate-cxx-headers ALL DEPENDS ${_all_includes})
+add_custom_target(generate-cxx-headers ALL DEPENDS ${_generated_includes})
 
 add_library(cxx-headers INTERFACE)
 target_link_libraries(cxx-headers INTERFACE libcxx-libc-headers libcxx-abi-headers)
@@ -1050,7 +1041,8 @@ add_dependencies(cxx-headers generate-cxx-headers)
 # It's important that the arch directory be included first so that its header files
 # which interpose on the default include dir be included instead of the default ones.
 target_include_directories(cxx-headers INTERFACE ${LIBCXX_GENERATED_INCLUDE_TARGET_DIR}
-                                                 ${LIBCXX_GENERATED_INCLUDE_DIR})
+                                                 ${LIBCXX_GENERATED_INCLUDE_DIR}
+                                                 ${CMAKE_CURRENT_SOURCE_DIR})
 
 if (LIBCXX_INSTALL_HEADERS)
   foreach(file ${files})
diff --git a/lldb/test/CMakeLists.txt b/lldb/test/CMakeLists.txt
index 6449ac5a9247f6..d51b18d71f3980 100644
--- a/lldb/test/CMakeLists.txt
+++ b/lldb/test/CMakeLists.txt
@@ -157,15 +157,22 @@ if(TARGET clang)
   # TestFullLtoStepping depends on LTO, and only runs when the compiler is clang.
   add_lldb_test_dependency(LTO)
 
-  if (TARGET libcxx OR ("libcxx" IN_LIST LLVM_ENABLE_RUNTIMES))
+  if ("libcxx" IN_LIST LLVM_ENABLE_RUNTIMES)
     set(LLDB_HAS_LIBCXX ON)
+    # libc++ can only be used for tests after installing to a fake prefix
+    # which is done by the install-cxx-test-suite-prefix target in libc++. We
+    # ensure this is run by depending on the runtimes-test-depends target.
+    # While this could build too many dependencies, it is currently the only
+    # test dependencies target exposed by the runtimes build.
+    add_lldb_test_dependency(runtimes-test-depends)
+    set(LIBCXX_TEST_INSTALL_DIR "${CMAKE_BINARY_DIR}/runtimes/runtimes-${LLVM_DEFAULT_TARGET_TRIPLE}-bins/libcxx/test-suite-install")
     if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT APPLE)
-      set(LIBCXX_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR}/${LLVM_DEFAULT_TARGET_TRIPLE})
-      set(LIBCXX_GENERATED_INCLUDE_DIR "${LLVM_BINARY_DIR}/include/c++/v1")
-      set(LIBCXX_GENERATED_INCLUDE_TARGET_DIR "${LLVM_BINARY_DIR}/include/${LLVM_DEFAULT_TARGET_TRIPLE}/c++/v1")
+      set(LIBCXX_LIBRARY_DIR ${LIBCXX_TEST_INSTALL_DIR}/lib/${LLVM_DEFAULT_TARGET_TRIPLE})
+      set(LIBCXX_GENERATED_INCLUDE_DIR "${LIBCXX_TEST_INSTALL_DIR}/include/c++/v1")
+      set(LIBCXX_GENERATED_INCLUDE_TARGET_DIR "${LIBCXX_TEST_INSTALL_DIR}/include/${LLVM_DEFAULT_TARGET_TRIPLE}/c++/v1")
     else()
-      set(LIBCXX_LIBRARY_DIR ${CMAKE_BINARY_DIR}/lib${LIBCXX_LIBDIR_SUFFIX})
-      set(LIBCXX_GENERATED_INCLUDE_DIR "${CMAKE_BINARY_DIR}/include/c++/v1")
+      set(LIBCXX_LIBRARY_DIR ${LIBCXX_TEST_INSTALL_DIR}/lib${LIBCXX_LIBDIR_SUFFIX})
+      set(LIBCXX_GENERATED_INCLUDE_DIR "${LIBCXX_TEST_INSTALL_DIR}/include/c++/v1")
     endif()
   endif()
 
diff --git a/lldb/utils/lldb-dotest/CMakeLists.txt b/lldb/utils/lldb-dotest/CMakeLists.txt
index 3b8c88b6dc78cb..414caf43df0e6c 100644
--- a/lldb/utils/lldb-dotest/CMakeLists.txt
+++ b/lldb/utils/lldb-dotest/CMakeLists.txt
@@ -14,13 +14,19 @@ llvm_canonicalize_cmake_booleans(
 
 if ("libcxx" IN_LIST LLVM_ENABLE_RUNTIMES)
   set(LLDB_HAS_LIBCXX ON)
+  # libc++ can only be used for tests after installing to a fake prefix
+  # which is done by the install-cxx-test-suite-prefix target in libc++. We
+  # ensure this is run by depending on the runtimes-test-depends target.
+  # While this could build too many dependencies, it is currently the only
+  # test dependencies target exposed by the runtimes build.
+  set(LIBCXX_TEST_INSTALL_DIR "${CMAKE_BINARY_DIR}/runtimes/runtimes-${LLVM_DEFAULT_TARGET_TRIPLE}-bins/libcxx/test-suite-install")
   if(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR AND NOT APPLE)
-    set(LIBCXX_LIBRARY_DIR ${LLVM_LIBRARY_OUTPUT_INTDIR}/${LLVM_DEFAULT_TARGET_TRIPLE})
-    set(LIBCXX_GENERATED_INCLUDE_DIR "${LLVM_BINARY_DIR}/include/c++/v1")
-    set(LIBCXX_GENERATED_INCLUDE_TARGET_DIR "${LLVM_BINARY_DIR}/include/${LLVM_DEFAULT_TARGET_TRIPLE}/c++/v1")
+    set(LIBCXX_LIBRARY_DIR ${LIBCXX_TEST_INSTALL_DIR}/lib/${LLVM_DEFAULT_TARGET_TRIPLE})
+    set(LIBCXX_GENERATED_INCLUDE_DIR "${LIBCXX_TEST_INSTALL_DIR}/include/c++/v1")
+    set(LIBCXX_GENERATED_INCLUDE_TARGET_DIR "${LIBCXX_TEST_INSTALL_DIR}/include/${LLVM_DEFAULT_TARGET_TRIPLE}/c++/v1")
   else()
-    set(LIBCXX_LIBRARY_DIR ${CMAKE_BINARY_DIR}/lib${LIBCXX_LIBDIR_SUFFIX})
-    set(LIBCXX_GENERATED_INCLUDE_DIR "${CMAKE_BINARY_DIR}/include/c++/v1")
+    set(LIBCXX_LIBRARY_DIR ${LIBCXX_TEST_INSTALL_DIR}/lib${LIBCXX_LIBDIR_SUFFIX})
+    set(LIBCXX_GENERATED_INCLUDE_DIR "${LIBCXX_TEST_INSTALL_DIR}/include/c++/v1")
   endif()
 endif()