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

[Hubert Tong via llvm-commits]

hubert-reinterpretcast wrote:

> LLVM Buildbot has detected a new failure on builder `ppc64-flang-aix` running on `ppc64-flang-aix-test` while building `flang,llvm,polly` at step 5 "build-unified-tree".
> 
> Full details are available at: https://lab.llvm.org/buildbot/#/builders/201/builds/7693
> Here is the relevant piece of the build log for the reference

@aengelke, thanks for the fix at https://github.com/llvm/llvm-project/commit/1ea49c77b36e8ecfc0053db137bf0d68f5021ba5. I am not sure why the problem came up though because we set macro definitions on the command line that should suppress the path in the AIX system header that results in the `S_RESERVED*` macros:
https://github.com/llvm/llvm-project/blob/9f6ad7654b07391525124edcab498c938da10e92/llvm/CMakeLists.txt#L1247-L1252

When the PCH is created, I would hope that the macro definitions we expect during compilation would be used.

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