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

[Alexis Engelke via cfe-commits]

aengelke wrote:

@bangtianliu Thanks for testing. I fixed the combination with `CMAKE_POSITION_INDEPENDENT_CODE=ON`. I couldn't reproduce the error regarding visibility. Can you provide a LLVM CMake invocation that leads to this error?

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