When an @_implementationOnly import includes Objective-C categories
for existing types, it's useful to be able to override the members
provided in those categories without exposing them to clients of the
framework being built. Allow this as long as the overriding
declaration is marked as @_implementationOnly itself, with an
additional check that the type of the declaration does not change.
(Normally overrides are allowed to change in covariant ways.)
Part of rdar://50827914