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

[Kajetan Puchalski via llvm-commits]

mrkajetanp wrote:

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

A fresh ccache log is showing cache hits for the precompiled headers with this patch just as without it, yes.

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