swift234

[Sema] When looking up member types, use associated types as a last resort.

Swift can infer associated types for decls conforming to protocols, but if
there's already an explicit member type available, we can just use that.

We could probably push this deeper (into qualified lookup), but this is
enough to avoid cases of accidental circular conformance checking, where
we need a particular member type to satisfy a parent conformance.

rdar://problem/18576085

Swift SVN r22664