swift234

Reapply two tweaks to if/let handling:

two logically independent but related patches conflated together: 
 - Improve error recovery for malformed if/let conditions, particularly 
   when the user uses "," instead of &&. Add testcases for error recovery 
   requested by Jordan.

- Add a syntactic requirement that the pattern of an if/let condition be 
  a simple identifier or _. We allow slightly broader patterns here now, 
  but they will change in the future when refutable patterns are allowed. 
  It is best to be narrow and then open it up later so we can do a great
  job of QoI then.

This includes the changes to the stdlib directory that I forgot to commit
with the patch the previous time.



Swift SVN r25408