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

[Alexis Engelke via cfe-commits]

aengelke wrote:

@boomanaiden154 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. I have no idea where the configuration lives and what I would need to do to pass `-DCMAKE_DISABLE_PRECOMPILE_HEADERS=ON`, can you give some pointers here?

The PR currently disables PCH for GCC, which means that post-commit builders will implicitly have coverage for both PCH and non-PCH builds.

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