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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

@mrkajetanp Did you check whether with these changes, ccache does cache those compilations?

I am using ccache under Windows and works fine. I could not reproduce the error in https://lab.llvm.org/staging/#/builders/206/builds/174. 

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