[libTooling] Fix Transformer to work with ambient traversal kinds.

Summary:
`RewriteRule`'s `applyFirst` was brittle with respect to the default setting of the
`TraversalKind`. This patch builds awareness of traversal kinds directly into
rewrite rules so that they are insensitive to any changes in defaults.

Reviewers: steveire, gribozavr

Subscribers: hokein, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D80606