In Diviner, parse "abstract" and "final" in PHP classes, interfaces and methods

Summary: Ref T988. Adds support for the "abstract" and "final" keywords in the atomizer.

Test Plan: Looked at abstract/final stuff.

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D6850