Improve performance of LINT_UNNECESSARY_SEMICOLON

Summary: Ref T7892. Improve the performance of ArcanistXHPASTLinter::LINT_UNNECESSARY_SEMICOLON.

Test Plan: Wiped 6 seconds off the total time to lint rARC.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7892

Differential Revision: https://secure.phabricator.com/D12519