swift234

[Clang importer] Make sure an unavailable superclass init doesn't override an available import-as-member-init.

More generally, an unavailable initializer shouldn't stomp on an
available initializer, because it's possible that (for example) a
designated initializer will be unavailable but a factory initializer
will be available, so one still construct objects of that type.
Fixes rdar://problem/26238032.