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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> @hubert-reinterpretcast The PCH should use the same command line flags as files in the LLVMSupport library. I don't have access to AIX systems, so I can't investigate what's going on.

Got it; thanks. I'll reach out privately to people who might be able to follow up.

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