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

[Alexis Engelke via cfe-commits]

aengelke wrote:

@DavidTruby Hmm, I don't think we support BUILD_SHARED_LIBS on non-mingw Windows? Could you try #182883?

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