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

[Bangtian Liu via cfe-commits]

bangtianliu 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?

should be here: https://github.com/iree-org/llvm-project/blob/1839675d168e357c3db1559446031d55833f0181/llvm/lib/Target/CMakeLists.txt#L25-L32

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