Fix brace formatting linter rule after XHPAST changes
Summary: Adjusts ArcanistBraceFormattingXHPASTLinterRule after changes to the way in which XHPAST parses namespaces. Depends on D14498.
Test Plan: Unit tests are now passing.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D14499