Instances of Blueprints implemented native interface cannot be assigned to interface variables

FInterfaceProperty::SerializeItem (and ImportText) attempt to determine if the passed in object implements the interface in question. They do this by calling GetInterfaceAddress, which tries to get a pointer to the interface portion of the object itself. There are 3 cases:

1. For a blueprint-defined interface, there is no C++ representation at all so it just returns the object
2. For a native-defined and native-implemented interface, it returns a pointer to the offset where the native-implemented interface lives
3. For a native-defined but blueprint-implemented interface, it returns nullptr because in this case the C++ representation is missing. This case is only possible for interfaces that don't actually have any BlueprintCallable functions

The case breaking here is #3, and serialization should probably use a different function to verify that it can be properly set. This code is quite old and predates TScriptInterface, which allows safely storing objects that bp-implement a native interface

1. Create a native interface IMyInterface and create a blueprint that implements that MyImplementer.
2. Create another blueprint MyReferencer that has an IMyInterface variable.
3. Add a MyReferencer instance and a MyImplementer instance to a level.
4. Try assigning the MyImplementer instance to the MyReferencer variable.
5. You get `InterfaceProperty /Game/Lighting/Blueprints/MyReferencer.MyReferencer_C:NewVar_0: specified object doesn't implement the required interface class 'IMyInterface': /Temp/Untitled_1.Untitled_1:PersistentLevel.MyImplementer_2`