Inform the consumer on invalid template instantiations.

[v.g.vassilev]

Summary

Some clients which want to track state need the information whether a template was instantiated and made invalid.

[rsmith]

Looks like the in-tree consumers are fine with being called on invalid declarations (CodeGen bails out if any errors have occurred). I don't think there's any easy way to find if out-of-tree consumers will be OK with this, but it does seem reasonable to ensure that we pass all the tag definitions to the consumer, even if they're invalid.

clang/lib/Sema/SemaTemplateInstantiate.cpp

for (TemplateDeclInstantiator::delayed_var_partial_spec_iterator
       }
     }
   }
 
   // Exit the scope of this instantiation.
   SavedContext.pop();
 
   if (!Instantiation->isInvalidDecl()) {
-    Consumer.HandleTagDeclDefinition(Instantiation);
-
     // Always emit the vtable for an explicit instantiation definition
     // of a polymorphic class template specialization. Otherwise, eagerly
     // instantiate only constexpr virtual functions in preparation for their use
     // in constant evaluation.
     if (TSK == TSK_ExplicitInstantiationDefinition)
       MarkVTableUsed(PointOfInstantiation, Instantiation, true);
     else if (MightHaveConstexprVirtualFunctions)
       MarkVirtualMembersReferenced(PointOfInstantiation, Instantiation,
                                    /*ConstexprOnly*/ true);
   }
 
+  Consumer.HandleTagDeclDefinition(Instantiation);
+
   return Instantiation->isInvalidDecl();
 }
 
 /// Instantiate the definition of an enum from a given pattern.
 ///
 /// \param PointOfInstantiation The point of instantiation within the
 ///        source code.
 /// \param Instantiation is the declaration whose definition is being