swift234

stdlib: improve performance of string appending

          CaptureProp ,  -21.8%
             HeapSort ,   12.7%
            ImageProc ,  -25.6%
               StrCat ,   92.6%
       StrComplexWalk ,   11.1%
             StrToInt ,   16.6%
  StringInterpolation ,   21.6%

Regression in CaptureProp is due to some interference of the harness.  When the
code is extracted into a separate file, there is no difference.

Regression in ImageProc is caused by unconditional construction of a string for
CheckResults()

rdar://18119872

Swift SVN r21535