Symmetry with what we did for Arrays says that Strings shoudl only
concatenate to Strings using "+". We have append() for adding single
characters.
Swift SVN r20997
<rdar://problem/17058606> for loop range variable shares name with iteration counter - that's wrong and misleading
<rdar://problem/16786106> [Xcode 6.1] foreach generators are always named "$generator"
Swift SVN r20683