[AST] Extract ASTNodeTraverser class from ASTDumper

Summary:
This new traverser class allows clients to re-use the traversal logic
which was previously part of ASTDumper.  This means that alternative
visit logic may be implemented, such as

* Dump to alternative data formats such as JSON
* Implement AST Matcher parent/child visitation matching AST dumps

Reviewers: aaron.ballman

Subscribers: jfb, cfe-commits

Differential Revision: https://reviews.llvm.org/D57472

llvm-svn: 352989