swift234

[stdlib] Fix reserve when extending String

I wish I knew how to code a regression test for this, but sadly I don't.

The only performance differences with magnitude > 7% show up in
-Ounchecked, and most of these look suspiciously like noise due to the
irrelevance of generic string append.  The only one I'd be remotely
concerned about is StrToInt, but it doesn't make any sense that it
should have gotten slower, so I think that's noise too.

Ackermann: -14.3%
Fibonacci: 33.3%
ImageProc: 9.0%
Phonebook: -7.1%
QuickSort: 15.6%
StrToInt: -14.3%

Swift SVN r21236