Convert some functions to use StringRef instead of c_str, len

This started as an effort to change StringExtractor to store a
StringRef internally instead of a std::string.  I got that working
locally with just 1 test failure which I was unable to figure out the
cause of.  But it was also a massive changelist due to a trickle
down effect of changes.

So I'm starting over, using what I learned from the first time to
tackle smaller, more isolated changes hopefully leading up to
a full conversion by the end.

At first the changes (such as in this CL) will seem mostly
a matter of preference and pointless otherwise.  However, there
are some places in my larger CL where using StringRef turned 20+
lines of code into 2, drastically simplifying logic.  Hopefully
once these go in they will illustrate some of the benefits of
thinking in terms of StringRef.

llvm-svn: 279917