Copy StringExtractor to StdStringExtractor.

I have some improvements to make to StringExtractor that require
using LLVM.  debugserver can't take a dependency on LLVM but uses
this file, so I'm forking it off into StdStringExtractor and
StringExtractor, so that StringExtractor can take advantage of
some performance improvements and readability improvements that
LLVM can provide.

llvm-svn: 279997