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

Fixing the standalone builds by exporting the vt_gen target so that the
sources of the clangCodeGen target can form an explicit dependency edge
on it.

(cherry picked from commit cace986)

Author: etcwilde

llvm/cmake/modules/CMakeLists.txt

@@ -36,6 +36,9 @@ endif()
 if(omp_gen IN_LIST LLVM_COMMON_DEPENDS)
   list(REMOVE_ITEM LLVM_COMMON_DEPENDS omp_gen)
 endif()
+if(vt_gen IN_LIST LLVM_COMMON_DEPENDS)
+  list(REMOVE_ITEM LLVM_COMMON_DEPENDS vt_gen)
+endif()
 
 #
 # Generate LLVMConfig.cmake for the build tree.

llvm/cmake/modules/LLVMConfig.cmake.in

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