Convert some helper functions to use StringRef.

I will probably submit a lot of small trivial changes
like this over the coming weeks.  The end goal is to
convert Options::SetOptionValue to take the option arg
as a StringRef, but doing so in one pass would be a huge
mess of disparate changes just to satisfy the compiler,
and with a high risk of breaking.  So I'm going to do
this in small pieces, changing seemingly random things
here and there until the final change to the signature
of Options::SetOptionValue() can be done trivially.

llvm-svn: 286088