swift234

[IRGen] Casts to an ObjC-class-bound archetype are not casts to the bound.

And they're not even guaranteed to be casts to ObjC classes. They might be
Swift subclasses of ObjC classes.

This fixes a type-safety hole where, e.g. a generic cast from NSPredicate
to NSDate was allowed because we were checking against NSObject.

rdar://problem/22242369

Swift SVN r31153