[LCG] Move the call graph node class into the graph class's definition.
This will become necessary to build up the SCC iterators and SCC
definitions. Moving it now so that subsequent diffs are incremental.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206454 91177308-0d34-0410-b5e6-96231b3b80d8