[GraphTraits] Add support for iterating over children edges.

Summary:
This change is mostly adding comments to GraphTraits describing
interfaces to iterate over children edges of a node. These will
have to be implemented by specializations of GraphTraits. The
non-comment change is the addition of children_edges template
function that returns an iterator range.

The motivation for this is to use it in synthetic count propagation
algorithm and remove the CallGraphTraits class that provide similar
interfaces.

Reviewers: dberlin, davidxl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D42698

llvm-svn: 323990