clang-rename: avoid StringRef members in USRLocFindingASTVisitor

[vmiklos]

Summary

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.

[klimek]

LG

clang-rename/USRLocFinder.cpp

while (NameLoc) {
       const auto *Decl = NameLoc.getNestedNameSpecifier()->getAsNamespace();
       if (Decl && getUSRForDecl(Decl) == USR)
         LocationsFound.push_back(NameLoc.getLocalBeginLoc());
       NameLoc = NameLoc.getPrefix();
     }
   }
 
   // All the locations of the USR were found.
-  StringRef USR;
+  const std::string USR;
   // Old name that is renamed.
-  StringRef PrevName;
+  const std::string PrevName;
   std::vector<clang::SourceLocation> LocationsFound;
 };
 } // namespace
 
 std::vector<SourceLocation> getLocationsOfUSR(StringRef USR,
                                               StringRef PrevName,
                                               Decl *Decl) {
   USRLocFindingASTVisitor visitor(USR, PrevName);
 
   visitor.TraverseDecl(Decl);
   return visitor.getLocationsFound();
 }
 
 } // namespace rename
 } // namespace clang