Re-allow indexing on "self" and object types in GDScript

Fixes #25081

(cherry picked from commit e3d05fa42d)

modules/gdscript/gdscript_parser.cpp

@@ -6444,7 +6444,7 @@ GDScriptParser::DataType GDScriptParser::_reduce_node_type(Node *p_node) {
 							return DataType();
 						}
 					}
-					if (check_types && !node_type.has_type) {
+					if (check_types && !node_type.has_type && base_type.kind == DataType::BUILTIN) {
 						// Can infer indexing type for some variant types
 						DataType result;
 						result.has_type = true;