swift234

Implement SILGen for multi-pattern conditions in if/while, finishing the 
known implementation work required for:
<rdar://problem/19382942> Improve 'if let' to avoid optional pyramid of doom

I want to write some more tests and improve error recovery QoI in the parser,
but I believe it fully works now.  Please kick the tires and let me know if
you observe any problems (other than 19432424 which was pre-existing).



Swift SVN r24348