[ADT] NFC: Generalize GraphTraits requirement of "NodeType *" in interfaces to...

[ADT] NFC: Generalize GraphTraits requirement of "NodeType *" in interfaces to "NodeRef", and migrate SCCIterator.h to use NodeRef

Summary: By generalize the interface, users are able to inject more flexible Node token into the algorithm, for example, a pair of vector<Node>* and index integer. Currently I only migrated SCCIterator to use NodeRef, but more is coming. It's a NFC.

Reviewers: dblaikie, chandlerc

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D22937

llvm-svn: 277399