AST: Update TypeBase::mayHaveSuperclass() for subclass existentials

Author: slavapestov

include/swift/AST/Types.h

@@ -4556,9 +4556,14 @@ inline bool TypeBase::mayHaveSuperclass() {
   if (getClassOrBoundGenericClass())
     return true;
 
+  // FIXME: requiresClass() is not the same as having an explicit superclass;
+  // is this wrong?
   if (auto archetype = getAs<ArchetypeType>())
     return (bool)archetype->requiresClass();
 
+  if (isExistentialType())
+    return (bool)getSuperclass(nullptr);
+
   return is<DynamicSelfType>();
 }