clang-rename: avoid StringRef members in USRLocFindingASTVisitor

Even if this is defined in the .cpp file and only used as part of the
function (so here it's safe), usually storing StringRefs in the class is
dangerous, so don't do so.

Reviewers: cfe-commits, klimek

Differential Revision: http://reviews.llvm.org/D20296

llvm-svn: 269796