[refactor] Describe refactorings in the operation classes

This commit changes the way that the refactoring operation classes are
structured:
- Users have to call `initiate` instead of constructing an instance of the
  class. The `initiate` is now supposed to have custom initiation logic, and
  you don't need to subclass the builtin requirements.
- A new `describe` function returns a structure with the id, title and the
  description of the refactoring operation.

The refactoring action classes are now placed into one common place in
RefactoringActions.cpp instead of being separate.

Differential Revision: https://reviews.llvm.org/D38985

llvm-svn: 316780