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

[Maksim Levental via cfe-commits]

https://github.com/makslevental commented:

Just my 2 cents: The CMake-isms look completely reasonable but I'll come back to take a closer look soon. But thank you for working on this - a %1 improvement in LLVM compile time probably saves 1000 trees per year or something like that 😄.

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