swift234

Change the convertFromStringLiteral convention to be a bit more efficient:
add an alternate entry-point called convertFromASCIIStringLiteral which is called for
string literals which contain only ASCII codepoints, and add an optional byteLength
argument for the conversion.  <rdar://problem/11764780>.

I've also tentatively changed swift.String to use the new convention; Dave,
please review.



Swift SVN r2290