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

[Kajetan Puchalski via llvm-commits]

mrkajetanp wrote:

Just to save people from clicking through the related PRs, here's the ccache documentation I based this off of:
https://ccache.dev/manual/latest.html#_precompiled_headers

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