[libTooling] Small changes in Transformer API.

Summary:
* Rename `transformer::change` to `transformer::changeTo`, make `change` forward
  to `changeTo` and mark it deprecated.

* Mark `transformer::text` and `transformer::selection` deprecated and migrate
  references to them in tests.

Reviewers: ilya-biryukov

Subscribers: gribozavr, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D69896