swift234

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.



Swift SVN r25371