swift234

These changes implement some oft-requested tweaks and fixes to our closure implementation:
- Closures that are comprised of only a single return statement are now considered to be "single expression" closures. (rdar://problem/17550847)
- Unannotated single expression closures with non-void return types can now be used in void contexts. (rdar://problem/17228969)
- Situations where a multi-statement closure's type could not be inferred because of the lack of a return-type annotation are now properly diagnosed. (rdar://problem/17212107)

I also encountered a number of crashers along the way, which should now be fixed.

Swift SVN r24817