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

[Jon Roelofs via cfe-commits]

jroelofs wrote:

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

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