swift234

Speculatively land the grammar change for <rdar://problem/20167393> revise typed-pattern grammar to make refutable patterns a superset of irrefutable ones

This is still a subject of discussion on swift-dev, but it seems like clearly the right
way to go to me.  If it turns out that this isn't a good direction, I'll revert this and 
subsequent patches built on top of it.



Swift SVN r26168