[objc-arc] Added debug statement saying when we are resetting a sequence's progress.

This will make it clearer when we are actually resetting a sequence's progress
vs just changing state. This is an important distinction because the former case
clears any pointers that we are tracking while the later does not.

llvm-svn: 179963

llvm/lib/Transforms/ObjCARC/ObjCARCOpts.cpp

@@ -472,6 +472,7 @@ namespace {
     }
 
     void ResetSequenceProgress(Sequence NewSeq) {
+      DEBUG(dbgs() << "Resetting sequence progress.\n");
       SetSeq(NewSeq);
       Partial = false;
       RRI.clear();