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

[Alexis Engelke via cfe-commits]

aengelke wrote:

Re build failure: it turns out that CMake disables warnings in PCH by default, I filed #176566 to work around this.

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