Runtime: More robust NSStringToString_slow.
Runtime: More robust NSStringToString_slow.

It still leaks malloced buffers, but it should now work with any NSString subclass, and it won't fail if the UTF-8 byte length is more than twice the surrogate-pair length.

Swift SVN r3921

apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation,apple/swift-corelibs-foundation