swift

54e6900c - Sema: Skip diagnosing witness availability for unavailable conformances.

Sema: Skip diagnosing witness availability for unavailable conformances. If a conformance is unavailable then checking for the potential unavailability of a witness doesn't make sense so skip it. This is necessary now because availability scopes no longer constrain the platform introduction version for a source range in that is annotated in source to be simultaneously unavailable and also introduced in a later version. This fixes a regression caused by https://github.com/swiftlang/swift/pull/79249. Resolves rdar://145255592.