[llvm] Export empty vt_gen target (for standalone builds) (PR #109817)

[Saleem Abdulrasool via llvm-commits]

================
@@ -151,6 +151,9 @@ endif()
 if(NOT TARGET intrinsics_gen)
   add_custom_target(intrinsics_gen)
 endif()
+if(NOT TARGET vt_gen)
----------------
compnerd wrote:

Ugh, leave this one be I suppose.

https://github.com/llvm/llvm-project/pull/109817