Record and represent environment variables

[pgoodman]

https://code.woboq.org/llvm/clang/lib/Driver/ToolChains/Clang.cpp.html#1334

// Parse additional include paths from environment variables.
--
// FIXME: We should probably sink the logic for handling these from the
// frontend into the driver. It will allow deleting 4 otherwise unused flags.
// CPATH - included following the user specified includes (but prior to
// builtin and standard includes).
addDirectoryList(Args, CmdArgs, "-I", "CPATH");
// C_INCLUDE_PATH - system includes enabled when compiling C.
addDirectoryList(Args, CmdArgs, "-c-isystem", "C_INCLUDE_PATH");
// CPLUS_INCLUDE_PATH - system includes enabled when compiling C++.
addDirectoryList(Args, CmdArgs, "-cxx-isystem", "CPLUS_INCLUDE_PATH");
// OBJC_INCLUDE_PATH - system includes enabled when compiling ObjC.
addDirectoryList(Args, CmdArgs, "-objc-isystem", "OBJC_INCLUDE_PATH");
// OBJCPLUS_INCLUDE_PATH - system includes enabled when compiling ObjC++.
addDirectoryList(Args, CmdArgs, "-objcxx-isystem", "OBJCPLUS_INCLUDE_PATH");

https://code.woboq.org/llvm/clang/lib/Driver/ToolChains/Darwin.cpp.html#1730

if (char *env = ::getenv("SDKROOT")) {
--
// We only use this value as the default if it is an absolute path,
// exists, and it is not the root path.
if (llvm::sys::path::is_absolute(env) && getVFS().exists(env) &&
StringRef(env) != "/") {
Args.append(Args.MakeSeparateArg(
nullptr, Opts.getOption(options::OPT_isysroot), env));
}
}

https://code.woboq.org/llvm/clang/tools/driver/driver.cpp.html#449

std::set<std::string> SavedStrings;
// Handle CCC_OVERRIDE_OPTIONS, used for editing a command line behind the
// scenes.
if (const char *OverrideStr = ::getenv("CCC_OVERRIDE_OPTIONS")) {
// FIXME: Driver shouldn't take extra initial argument.
ApplyQAOverride(argv, OverrideStr, SavedStrings);
}

etc.