Add support for Objective-C method implementation with Odin calling convention.

Use @objc_context_provider to provide a context for a type.

src/entity.cpp

@@ -238,6 +238,7 @@ struct Entity {
             bool   objc_is_implementation;
             Type*  objc_superclass;
             Type*  objc_ivar;
+			Entity*objc_context_provider;
 			String objc_class_name;
 			TypeNameObjCMetadata *objc_metadata;
 		} TypeName;