swift234

AST: Cleanups for TypeBase::getSuperclass()

First, enforce that the superclass of a class is an interface type.
Previously, Swift classes used interface types but imported
Objective-C generics used archetypes.

When the superclass type is always an interface type, we
can use the recently-added gatherAllSubstitutions() instead of
rolling our own parent type walk.

Also, this exposed an issue in name lookup where we would call
getSuperclass() on a type whose parent was an unbound generic.
This doesn't make sense, so generalize the existing check there.