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

[mgrang]

Summary

This fixes the following failure uncovered by D39245:

Clang :: Index/getcursor-preamble.m

[mgrang]

tools/libclang/CIndex.cpp

1028 ↗

(On Diff #124798)

Also removed the assert from here since isBeforeInTranslationUnit already has the same assert.

[arphaman]

Thanks, LGTM

cfe/trunk/tools/libclang/CIndex.cpp

for (auto *SubDecl : D->decls()) {
     DeclsInContainer.push_back(SubDecl);
   }
 
   // Now sort the Decls so that they appear in lexical order.
   std::sort(DeclsInContainer.begin(), DeclsInContainer.end(),
             [&SM](Decl *A, Decl *B) {
     SourceLocation L_A = A->getLocStart();
     SourceLocation L_B = B->getLocStart();
-    assert(L_A.isValid() && L_B.isValid());
-    return SM.isBeforeInTranslationUnit(L_A, L_B);
+    return L_A != L_B ?
+           SM.isBeforeInTranslationUnit(L_A, L_B) :
+           SM.isBeforeInTranslationUnit(A->getLocEnd(), B->getLocEnd());
   });
 
   // Now visit the decls.
   for (SmallVectorImpl<Decl*>::iterator I = DeclsInContainer.begin(),
          E = DeclsInContainer.end(); I != E; ++I) {
     CXCursor Cursor = MakeCXCursor(*I, TU, RegionOfInterest);
     const Optional<bool> &V = shouldVisitCursor(Cursor);
     if (!V.hasValue())