Previously, when an attempt was made to instantiate a generic metadata
whose argument was constrained to subclass a superclass Super with an
existential type E that had a superclass constraint to a subclass Sub of
that same superclass Super, the instantiation would fail at runtime,
despite the fact that the generic instantiation was allowed to
type-check.
The result was a runtime failure to instantiate generic metadata
resulting eventually in a crash.
Here, handling for that situation is added. When checking generic
requirements at runtime, when a superclass requirement is encountered,
an existential type is checked for. If that existential type has a
superclass constraint and if that superclass constraint is a subclass of
the superclass requirement, the check is determined to be satisfactory.
rdar://problem/64672291
