[ffigen] Filter copied methods

[liamappelbe]

There are a few ways that methods can be copied from one ObjC interface/protocol to another (eg copying methods from a protocol to a conforming interface). This bypasses method filtering (the filtering is applied to Source.method, not Dest.method). After the transformer refactor, we should do this method copying as a transformer, before the filtering.

[liamappelbe]

Fixed by #1681