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.

The `neg` test from SI-6162 has been split into a pos test (subclasses in the same compilation unit), and a neg test (subclasses in a different compilation unit.)

Review by @JamesIry.