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

[Alexis Engelke via cfe-commits]

aengelke wrote:

@DavidTruby Can you provide full build logs, preferably including the command invocations for compiling the PCH and the affected source files?

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