[ADT] Migrate DepthFirstIterator to use NodeRef

Summary:
Notice that the data layout is changed: instead of using
std::pair<PointerIntPair<NodeType*, 1>, ChildItTy>, now use
std::pair<NodeRef, Optional<ChildItTy>>.

A NFC but worth noticing change is operator==(), since we only compare
an iterator against end(), it's better to put an assert there and make
people noticed when it fails.

Reviewers: dblaikie, chandlerc

Subscribers: mzolotukhin, llvm-commits

Differential Revision: https://reviews.llvm.org/D23146

llvm-svn: 278437