swift234

[Constraint solver] Explore types that conform to each required protocol.

Keep track of each of the nominal type declarations and extensions
thereof that conform to each protocol. When the type checker runs out
of other ideas for a type variable, enumerate the types known to
conform to the protocols it requires. Fixes <rdar://problem/14014895>
and eliminates the extra casts introduced in r5639.


Swift SVN r5645