swift234

Progress towards let/else:
 - Implement SILGen for conditional multi-pattern PBD's.
 - Have the type checker check where clauses on PBDs.
 - Change the AST to represent complex if/let PBD's with
   composed PBDs instead of breaking them down. For example, 
   represent:
     if let x? = foo(), y? = bar() where x == y {
   as a single PBD in a StmtCondition instead of representing
   it as three entries in the condition.

The later change is good for AST/source fidelity as well as providing
a cheap way to exercise all the logic I'm building.



Swift SVN r26959