[DomTree] Replace ChildrenGetter with GraphTraits over GraphDiff.

Summary:
This replaces the ChildrenGetter inside the DominatorTree with
GraphTraits over a GraphDiff object, an object which encapsulated the
view of the previous CFG.
This also simplifies the extentions in clang which use DominatorTree, as
GraphDiff also filters nullptrs.

Reviewers: kuhar, dblaikie, NutshellySima

Subscribers: hiraditya, cfe-commits, llvm-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77341