[NFC][Clang Improve performance of DoMarkVarDeclReferenced (#161648)

Address post commit feedback from #161231

Author: cor3ntin

clang/lib/Sema/SemaExpr.cpp

@@ -20107,9 +20107,10 @@ static void DoMarkVarDeclReferenced(
   bool NeededForConstantEvaluation =
       isPotentiallyConstantEvaluatedContext(SemaRef) && UsableInConstantExpr;
 
-  bool NeedDefinition = OdrUse == OdrUseContext::Used ||
-                        NeededForConstantEvaluation ||
-                        Var->getType()->isUndeducedType();
+  bool NeedDefinition =
+      OdrUse == OdrUseContext::Used || NeededForConstantEvaluation ||
+      (TSK != clang::TSK_Undeclared && !UsableInConstantExpr &&
+       Var->getType()->isUndeducedType());
 
   assert(!isa<VarTemplatePartialSpecializationDecl>(Var) &&
          "Can't instantiate a partial template specialization.");