[GraphWriter] Change GraphWriter to use NodeRef in GraphTraits

Summary:
This is part of the "NodeType* -> NodeRef" migration. Notice that since
GraphWriter prints object address as identity, I added a static_assert on
NodeRef to be a pointer type.

Reviewers: dblaikie

Subscribers: llvm-commits, MatzeB

Differential Revision: https://reviews.llvm.org/D23580

llvm-svn: 278966