[flang] [llvm] [CMake] Fix using precompiled headers with ccache (PR #131397)

[David Truby via llvm-commits]

https://github.com/DavidTruby approved this pull request.

I've done a little investigation and the reason this flag isn't default anyway when using .pch files is essentially to work around/protect against dodgy build systems that won't rebuild the pch properly in all cases when the .h has changed. So clang will error if the .pch hasn't been updated properly.

We can rely on CMake handling this correctly; the target has a dependency on the .pch and the .pch has a dependency on the .h so there will be no issue here. So I think it's fine to just add this flag in every case even when ccache/other caching software isn't being used.

LGTM on this basis but please wait for the other reviewers to comment to check that they agree. Thanks for the fix!

https://github.com/llvm/llvm-project/pull/131397