[flang] [llvm] [CMake] Support using precompiled headers with ccache in flang (PR #136856)

[Kajetan Puchalski via llvm-commits]

mrkajetanp wrote:

It's got 3 approvals and this code has already been reviewed as part of the previous PR, so I'll go ahead and try merging it to see how the build bots like it. If something goes wrong I'll have the revert button at the ready again.

https://github.com/llvm/llvm-project/pull/136856