During protocol witness matching for a protocol requirement with
`@differentiable` attributes, implicit `@differentiable` attributes may be
created for the witness under specific conditions (when the witness has a
`@differentiable` attribute with superset differentiability parameters, or
when the witness has less-than-public visibility).
Do not generate implicit `@differentiable` attributes for protocol witnesses
when the protocol conformance is declared from a separate file or type context
from the witness.
Resolves SR-13455.
