swift234

stdlib: fix some bugs in UTF-16/UTF-8 transcoder that is hidden inside String
implementation:

- don't crash on NSStrings that contain isolated surrogates (partial fix, there
  is a different code path that dispatches CoreFoundation and it still
  crashes);

- insert U+FFFD according to Unicode recommendation;

- fix decoding of surrogate pairs when the internal buffer is almost full
  (we used to drop characters in that case, rdar://16833733 + dups).


Swift SVN r19147