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

[Alexis Engelke via llvm-commits]

aengelke 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.

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