[clang] [Clang] Replace vt_gen with LLVMCodeGenTypes (PR #109601)

[Evan Wilde via cfe-commits]

etcwilde wrote:

This drops the dependency edge between the compile step and generating `GenVT.inc` though. There was already a (indirect) dependency between the link step of `clangCodeGen` and generating `GetnVT.inc`.
Perhaps LLVM should export `vt_gen` like it does `intrinsics_gen` so that standalone builds can pick it up?

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