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

[Alexis Engelke via cfe-commits]

aengelke wrote:

Thanks for investigating. For me, Clang 17 is broken but Clang 18 works, so disabling for Clang <18 by default.

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