[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