clang coverage test compile error with precompile headers

[yestyle]

Hi,

Similar to #1652, I got a build error when a combination of clang + test coverage + precompile headers:

sccache: encountered fatal error
sccache: error: failed to zip up compiler outputs
sccache: caused by: failed to zip up compiler outputs
sccache: caused by: failed to open file `"/home/yestyle/code/sccache-clang-pch/build_clang/CMakeFiles/main.dir/cmake_pch.hxx.gcno"`
sccache: caused by: failed to open file `/home/yestyle/code/sccache-clang-pch/build_clang/CMakeFiles/main.dir/cmake_pch.hxx.gcno`
sccache: caused by: No such file or directory (os error 2)

sccache version is 0.8.2 and no issues when building with g++.

Minimum reproducible example can be found in https://github.com/yestyle/sccache-clang-pch (build steps in build.sh).

Also similar to the fix for #1652, I tested a local build of sccache by changing optional to true here and it works. Not sure if there are any better solutions. Thanks.