swift234

stdlib/String: strength-reduce some String methods: prefer StringCore.append
over String.extend

Performance testsuite changes over 2% are as follows.  A lot of tests unrelated
to String have been affected because of measurement noise and because strings
are used in result verification.

                 Ary3     4.8%
          CaptureProp    10.3%
           Dictionary    -4.0%
         EditDistance     2.8%
               Forest    -4.0%
                 Hash     8.6%
        InsertionSort     6.7%
                 Life    -2.3%
               MatMul     2.6%
           NestedLoop     3.9%
        PopFrontArray     2.9%
             PrimeNum     3.6%
                Prims     2.2%
              SmallPT    -2.1%
               TwoSum     4.1%

Swift SVN r21629