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

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

Reviewers: rjmccall, rsmith

Reviewed By: rjmccall

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D39944

llvm-svn: 318121