Move TraversalKind enum to ast_type_traits

Summary:
Make it usable outside of ASTMatchFinder.  This will make it possible to
use this enum to control whether certain implicit nodes are skipped
while AST dumping for example.

Reviewers: klimek, aaron.ballman

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61836

llvm-svn: 360920