[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@179963 91177308-0d34-0410-b5e6-96231b3b80d8