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

[Kajetan Puchalski via llvm-commits]

mrkajetanp wrote:

@tblah @DavidTruby @mgorny @Meinersbur for review.

I'm not sure whether this needs special handling for Windows w.r.t. that `Ccache configuration through CMake is not supported on Windows. Please use environment variables.` line or not.

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