swift234

SIL: Emit String-to-NSString conversions.

If -nsstring-is-string is enabled, lower Strings in cc(c) and cc(objc) function types to NSString, and when calling them, insert calls to StringToNSString/NSStringToString to perform the bridging conversion.

This isn't quite ready for prime-time yet, because we still need to emit the inverse bridging for ObjC method thunks, and I haven't tested the IRGen end of things yet.

Swift SVN r5355