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

[Jakub Kuderski via llvm-commits]

kuhar wrote:

Err, how is it going to interact with downstream projects that don't disable PCH for C files?

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