SI-7312 @deprecatedInheritance now ignores same-file subclasses

This allows us to deprecate external inheritances as a prelude
to sealing a class, without enduring the warnings ourselved in
interlude.
SI-7315 Test @deprecatedInheritance / @specialized interplay

Don't warn about the specialized subclass.

Fixed in the previous commit for a similar issue SI-7312.