[ffigen] ObjC protocol methods with `ns_consumes_self`

Can ObjC protocol methods be annotated with `__attribute__((ns_consumes_self))`? How should we handle this edge case?

Start with ffigen's Objective-C protocol handling and any existing tests for protocol method attributes. Determine whether ns_consumes_self is valid in this context and what behavior is intended; done means the behavior is decided and covered by an appropriate regression test.

Written by the indexing model from the issue text.