[ADT] Fix DepthFirstIterator's std::iterator base to have normal typedefs

Summary: This is similiar to r278752, where I found that the std::iterator<...> base can be normal.

Reviewers: dblaikie

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D23527

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@278753 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed