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

[Alexis Engelke via cfe-commits]

aengelke wrote:

[c-t-t](https://llvm-compile-time-tracker.com/compare_clang.php?from=052fb00f24990dda3cc76f55c04a1c2745c311af&to=2f09f6eb5f58cd59a2b6f58895bfff774bf41de7&stat=instructions%3Au&sortBy=absolute-difference), in total -15% instructions:u/wall-time on stage2-clang.

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