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

[Vitaly Buka via llvm-commits]

vitalybuka wrote:

And here https://lab.llvm.org/buildbot/#/builders/169/builds/20166


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