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

[Alexis Engelke via cfe-commits]

aengelke wrote:

@dwblaikie Someone on Discourse was, yes. I suspect you have a compile_commands.json file in your source tree? Such files get picked up too aggressively by Clang tools also they also too aggressively match one of the entries with these tests..

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