swift234

Fix single-arg-for-tuple-param check

In matchTypes(), we check whether a type variable representing an
argument could potentially be bound to a single param in a TupleType
rather than the whole TupleType, but this check has been pretty
heuristic and ad hoc so far. Replace with an explicit check to see if
it's possible for exactly one of the parameters in the tuple type can be
bound to a single unlabeled argument.

<rdar://problem/22913570>