swift

5ecfee76 - [ConformanceLookup] Don't allow skipping inherited unavailable conformances

[ConformanceLookup] Don't allow skipping inherited unavailable conformances in favor of explicit available ones. The type checker does not support the notion of multiple protocol conformances; there can only be one conformance, and if that conformance is unavailable, you cannot specify your own available conformance. This is important for Sendable checking; if a framework specifies that a type is explicitly not Sendable with an unavailable Sendable conformance, clients cannot ignore Sendable violations involving that type. If a superclass wants to allow subclasses to add a Sendable conformance, it should not declare an unavailable Sendable conformance. (cherry picked from commit 7356fe8c8a1607358c98bfb22c20a2b413563ec1)