This replaces the logic inChildrenGetter 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 nullptrsThe simplification step removing the extensions in clang which use DominatorTree and also filters for nullptr are deferred to a subsequent cleanup patch.