[LibTooling] Add Explanation parameter to `makeRule`.

Summary:
Conceptually, a single-case RewriteRule has a matcher, edit(s) and an (optional)
explanation. `makeRule` previously only took the matcher and edit(s). This
change adds (optional) support for the explanation.

Reviewers: ilya-biryukov

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D62390

llvm-svn: 361643