Previously, the various generic builders implemented the methods
addGenericArgument and addGenericWitnessTable without being provided
what argument or witness table they were to add (because it was not
previously needed). Now, the GenericRequirement is passed along to both
methods.
We don't need to substitute the superclass type as we walk up a
class hierarchy, or look up the generic parameters and conformances
to check if they're concrete, since we're always just emitting
null pointers in place of the generic parameters and requirements,
to be filled at runtime.
Also, don't leave space for generic parameters and requirements from
Objective-C superclasses, since that's not how they're represented.
