[refactor] Simplify the refactoring interface

This commit simplifies the interface for the refactoring action rules and the
refactoring requirements. It merges the selection constraints and the selection
requirements into one class. The refactoring actions rules must now be
implemented using subclassing instead of raw function / lambda pointers. This
change also removes a bunch of template-based traits and other
template definitions that are now redundant.

Differential Revision: https://reviews.llvm.org/D37681

llvm-svn: 314704