[analyzer] Use PointerUnion to implement ExplodedNode::NodeGroup.

We shouldn't be reinventing our own wheels. This also paves the way for
marking different kinds of sinks.

No functionality change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162154 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed