[SourceLocations] Use stronger sort predicate to remove non-deterministic ordering

Summary:
This fixes the following failure uncovered by D39245:
  Clang :: Index/getcursor-preamble.m

Reviewers: gbenyei, akyrtzi, bkramer, arphaman

Reviewed By: arphaman

Subscribers: arphaman, cfe-commits

Differential Revision: https://reviews.llvm.org/D40618

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@319357 91177308-0d34-0410-b5e6-96231b3b80d8