Sema: Always allow method overrides to be as available as the context.
When a method override is as available as the class it's a member of, then it
can't be any more available. It doesn't make sense to diagnose such a method as
less available than the method it overrides. This regressed recently for
methods belonging to classes that are nested inside extensions. The
availability of the derived class may be defined by its context, but the
compiler was only checking the availability attributes directly on the class.
Resolves rdar://143600638.