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

[Baranov Victor via cfe-commits]

vbvictor wrote:

> Regarding the pre-commit CI: sccache appears to not support PCH and I think it would be a good idea to disable PCH in the pre-commit CI to catch missing includes.

Can we instead set `CMAKE_DISABLE_PRECOMPILE_HEADERS=ON` inside `llvm/CMakeLists.txt` if we see that 
`CMAKE_CXX_COMPILER_LAUNCHER` is set too `sccache`?
I use `sccache` locally, and currently, I will need to set `DISABLE_PRECOMPILE_HEADERS` manually, right? I think we should give a warning message if `sccache` + `pch` is activated.


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