Enhancements for the DynTypedNode

Added ASTNodeKind as a standalone way to represent node kinds and their hierarchy.
This change is to support ongoing work on D815.

Reviewers: klimek

CC: cfe-commits

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@184331 91177308-0d34-0410-b5e6-96231b3b80d8