AST: Extended types of extensions should be interface types

lib/Serialization/Serialization.cpp

@@ -2704,6 +2704,7 @@ void Serializer::writeDecl(const Decl *D) {
     auto contextID = addDeclContextRef(extension->getDeclContext());
     Type baseTy = extension->getExtendedType();
     assert(!baseTy->hasUnboundGenericType());
+    assert(!baseTy->hasArchetype());
 
     // FIXME: Use the canonical type here in order to minimize circularity
     // issues at deserialization time. A known problematic case here is