[refactor] initial support for refactoring action rules

This patch implements the initial support for refactoring action rules. The
first rule that's supported is a "source change" rule that returns a set of
atomic changes. This patch is based on the ideas presented in my RFC:

http://lists.llvm.org/pipermail/cfe-dev/2017-July/054831.html

The following pieces from the RFC are added by this patch:

- `createRefactoringRule` (known as `apply` in the RFC)
- `requiredSelection` refactoring action rule requirement.
- `selection::SourceSelectionRange` selection constraint.

Differential Revision: https://reviews.llvm.org/D36075

llvm-svn: 311884