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

[Alexis Engelke via cfe-commits]

aengelke wrote:

@AaronBallman Could you try adding `set_source_files_properties(ConstantFolding.cpp PROPERTIES SKIP_PRECOMPILE_HEADERS ON)` in llvm/lib/Analysis/CMakeLists.txt:32 (after the /fp flag is set) and see whether this fixes the problem?

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