Rename PhutilRealnameContextFreeGrammar for consistency

Summary: Ref T5655. Depends on D11226.

Test Plan: See D11226.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley

Maniphest Tasks: T5655

Differential Revision: https://secure.phabricator.com/D11227