swift234

Sema: Nuke getGenericTypeContextDepth()

Now that ConstraintSystem::openGeneric() is the only remaining
caller of this function, inline it in there and open-code the
logic.

Also, add a hack for opening nominal types contained inside
protocol types. This is invalid, but we should not crash, so
bind the type variable for the protocol 'Self' type to the
'Self' archetype, since it will not be equated with anything
otherwise.