[clang] [flang] [llvm] [mlir] [polly] [CMake][LLVM] Add PCH infrastructure and LLVMSupport PCH (PR #176420)

[Maksim Levental via cfe-commits]

makslevental wrote:

> I'm unsure on this point. sccache is a common CI configuration, and nobody is going to notice those warnings. I guess this depends somewhat on what the failure mode for sccache + pch is. Build failures? Silent miscompiles? Silent failures to use the cache?

Fair enough. I was thining about this and probably this one is fine because PCH either works on your system or doesn't (it's the `CMAKE_MODULE_LINKER_FLAGS`-type flags that are a problem where it should be determined on a target by target basis). 



https://github.com/llvm/llvm-project/pull/176420