Detect and correct "final private" methods in lint
Ref T13588. Marking a method "final private" has never been meaningful, and is an error in PHP8.
Add static analysis to detect (and correct) this issue.
Test Plan: Added unit tests, will lint "phabricator/".
Maniphest Tasks: T13588
Differential Revision: https://secure.phabricator.com/D21539