swift234

Take more steps towards allowing associated types referenced within constraints to conform to concrete types.  This allows me to lift some restrictions I added a couple of weeks ago, and addresses rdar://problem/16333042 and rdar://problem/16284599.

(These changes also address the type-check aspects of rdar://problem/16369105, but there are still some SIL generation issues that I need to work through before I can call that work done.)

Swift SVN r15337