[Clang Tablegen] Fix build when std::string::end() is not assignable

[orivej]

Summary

This fixes Clang error: expression is not assignable:

Result.erase(--Result.end());                                                        
             ^ ~~~~~~~~~~~~

utils/TableGen/ClangDiagnosticsEmitter.cpp

void VisitSelect(SelectPiece *P) {
     Result += getModifierName(P->ModKind);
     if (P->ModKind == MT_Select) {
       Result += "{";
       for (auto *D : P->Options) {
         Visit(D);
         Result += '|';
       }
       if (!P->Options.empty())
-        Result.erase(--Result.end());
+        Result.pop_back();
       Result += '}';
     }
     addInt(mapIndex(P->Index));
   }
 
   void VisitPlural(PluralPiece *P) {
     Result += "%plural{";
     assert(P->Options.size() == P->OptionPrefixes.size());
     for (unsigned I = 0, End = P->Options.size(); I < End; ++I) {
       if (P->OptionPrefixes[I])
         Visit(P->OptionPrefixes[I]);
       Visit(P->Options[I]);
       Result += "|";
     }
     if (!P->Options.empty())
-      Result.erase(--Result.end());
+      Result.pop_back();
     Result += '}';
     addInt(mapIndex(P->Index));
   }
 
   void VisitDiff(DiffPiece *P) {
     Result += "%diff{";
     Visit(P->Options[0]);
     Result += "|";