In cases where a subclass was unable to synthesize any initializers
(for example, because there were missing designated initializers in the
superclass), we were skipping checking of superclass required
initializers. This meant that we would silently accept subclasses that
cannot be initialized directly (that would produce an error), but
could at runtime be initialized via a required initializer... that
didn't account for the subclass.
Fixes the original problem from https://github.com/apple/swift/issues/69965,
but not the most minimal one.
