[GraphDiff] Use class method getChildren instead of GraphTraits.

Summary:
Use getChildren() method in GraphDiff instead of GraphTraits.

This simplifies the code and allows for refactorigns inside GraphDiff.
All usecase need not have a light-weight/copyable range.
Clean GraphTraits implementation.

Reviewers: dblaikie

Subscribers: hiraditya, llvm-commits, george.burgess.iv

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D84562