[clang] [Clang] Include LLVM CodeGen CMake file (PR #109601)

[Tristan Ross via cfe-commits]

RossComputerGuy wrote:

> The `add_subdirectory` is definitely odd and I'm not sure if that is a proper solution.

I'm not familiar with how LLVM CodeGen works and I am a bit new to LLVM but I looked at [`clang/CMakeLists.txt`](https://github.com/llvm/llvm-project/blob/d61b2590f8e360695a5298311855c8649337969f/clang/CMakeLists.txt#L111-L123) and so I basically just copied this `add_directories`.

https://github.com/llvm/llvm-project/pull/109601