Classes using the type-erased Objective-C generics model are represented in parts of IRGen as UnboundGenericTypes, which is a problem because a number of code paths expect all generic types to be bound. Update some of these that are involved in extensions on ObjC generic types.
In-place initialization means the class has a symbol we can reference
from the category, so there's nothing to do on the IRGen side.
For JIT mode, we just need to realize the class metadata by calling an
accessor instead of directly referencing the symbol though.
