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

[Evan Wilde via llvm-commits]

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

Do you want me to re-order all of them? `acc_gen` comes after `omp_gen` and `intrinsics_gen` in this list.

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