Detect and correct "final private" methods in lint

Summary:
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