swift234

Allow subclasses of specific instantiations of ObjC generic classes.

If a subclass grounds all the type parameters from its base class, we don't have to worry about any erasure edge cases. We should be able to support this, giving existing code that subclasses the nongeneric form of the class a migration path. Spot-fix some places in IRGen where we assume we can't emit static references to ObjC generic classes or metaclasses.