[Sema] Stable sort OverloadCandidates to remove non-deterministic ordering

[mgrang]

Summary

This fixes failure in Misc/diag-template-diffing.cpp uncovered by D39245.

[rjmccall]

Hmm. It looks like the intent is for CompareOverloadCandidatesForDisplay to be a total order, but I'm sure there are cases where it isn't. Okay, LGTM.

lib/Sema/SemaOverload.cpp

else if (OCD == OCD_AllCandidates) {
       CompleteNonViableCandidate(S, Cand, Args);
       if (Cand->Function || Cand->IsSurrogate)
         Cands.push_back(Cand);
       // Otherwise, this a non-viable builtin candidate.  We do not, in general,
       // want to list every possible builtin candidate.
     }
   }
 
-  std::sort(Cands.begin(), Cands.end(),
+  std::stable_sort(Cands.begin(), Cands.end(),
             CompareOverloadCandidatesForDisplay(S, OpLoc, Args.size(), Kind));
 
   bool ReportedAmbiguousConversions = false;
 
   SmallVectorImpl<OverloadCandidate*>::iterator I, E;
   const OverloadsShown ShowOverloads = S.Diags.getShowOverloads();
   unsigned CandsShown = 0;
   for (I = Cands.begin(), E = Cands.end(); I != E; ++I) {