swift

e28670d9 - [ConformanceLookup] Just kidding, the compiler needs to prefer available

[ConformanceLookup] Just kidding, the compiler needs to prefer available Sendable conformances for source compatibility. If conformance lookup always prefers the conformance from the defining module, libraries introducing unavailable Sendable conformances can break source in clients that declare retroactive Sendable conformances. Instead, still prefer the available conformance, and always diagnose the client conformance as redundant (as a warning). Then, when the retroactive conformance is removed, the errors will surface, so the programmer has to take explicit action to experience the source break. (cherry picked from commit b1397703a5ab3f1569079dd85e51eb6d67b3a5cf)