Remove CallGraphTraits and use equivalent methods in GraphTraits

Summary:
D42698 adds child_edge_{begin|end} and children_edges to GraphTraits
which are used here. The reason for this change is to make it easy to
use count propagation on ModulesummaryIndex. As it stands,
CallGraphTraits is in Analysis while ModuleSummaryIndex is in IR.

Reviewers: davidxl, dberlin

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D42703

llvm-svn: 323994