swift234

Enable string-to-pointer conversions and remove CString.

There is some follow-up work remaining:

- test/stdlib/UnicodeTrie test kills the type checker without manual type annotations. <rdar://problem/17539704>
- test/Sema/availability test raises a type error on 'a: String == nil', which we want, but probably not as a side effect of string-to-pointer conversions. I'll fix this next.

Swift SVN r19477