A few minor stylistic cleanups in StringExtractor.

A few minor stylistic cleanups in StringExtractor.

Makes Peek() return a StringRef instead of a const char*.

This leads to a few callers of Peek() being able to be made a
little nicer (for example using StringRef member functions instead
of c-style strncmp and related functions) and generally safer
usage.

llvm-svn: 280139