Make it possible control matcher traversal kind with ASTContext

Summary:
This will eventually allow traversal of an AST while ignoring invisible
AST nodes.  Currently it depends on the available enum values for
TraversalKinds.  That can be extended to ignore all invisible nodes in
the future.

Reviewers: klimek, aaron.ballman

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61837