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

[Saleem Abdulrasool via cfe-commits]

compnerd wrote:

> @compnerd @etcwilde WDYT about conditionalizing the new dependency with an `if (TARGET vt_gen)` instead of doing the `add_subdirectory()` dance?

Ugh, I misread the change as `include_directories`. The `add_subdirectory` is definitely odd and I'm not sure if that is a proper solution.

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