[ffigen] Bug in ObjC category filtering

[liamappelbe]

Methods on categories that are copied to their corresponding interface (eg due to returning intstancetype) bypass category filtering. There are a bunch of cases in package:objective_c where methods are present on interfaces from categories that haven't been included:

• NSArrayCreation
• NSDateCreation
• NSDictionaryCreation
• NSMutableArrayCreation
• NSMutableDictionaryCreation
• NSMutableSetCreation
• NSSetCreation
• NSStringCreation