GraphTraits: Add range versions of graph traits functions (graph_nodes,...

GraphTraits: Add range versions of graph traits functions (graph_nodes, graph_children, inverse_graph_nodes, inverse_graph_children).

Summary:
Convert all obvious node_begin/node_end and child_begin/child_end
pairs to range based for.

Sending for review in case someone has a good idea how to make
graph_children able to be inferred. It looks like it would require
changing GraphTraits to be two argument or something. I presume
inference does not happen because it would have to check every
GraphTraits in the world to see if the noderef types matched.

Note: This change was 3-staged with clang as well, which uses
Dominators/etc from LLVM.

Reviewers: chandlerc, tstellarAMD, dblaikie, rsmith

Subscribers: arsenm, llvm-commits, nhaehnle

Differential Revision: https://reviews.llvm.org/D29767

llvm-svn: 294620