Use explicit StringRef Type

We previously used a Twine here, but as pointed out by David Blaikie
and Mehdi Amini storing a temporary StringRef in a Twine is not a good
idea, as the StringRef will be freed before the Twine is used leaving
a Twine that points to uninitialized memory. We now make it explicit that
we use a StringRef here.

llvm-svn: 225342