[LibTooling] Add support to Transformer for composing rules as an ordered choice.

This revision updates `RewriteRule` to support multiple subrules that are
interpreted as an ordered-choice (apply the first one that matches). With this
feature, users can write the rules that appear later in the list of subrules
knowing that previous rules' patterns *have not matched*, freeing them from
reasoning about those cases in the current pattern.

Reviewers: ilya-biryukov

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D61335

llvm-svn: 361037