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

[David Truby via cfe-commits]

DavidTruby wrote:

@aengelke that patch seems to make some but not all of them go away. I still get the `CLANG_BUILD_STATIC` and `MLIR_BUILD_STATIC` ones but not the `LLVM_BUILD_STATIC` ones.

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