Fix a "continue;" inside a switch in ArcanistPHPCompatibilityXHPASTLinterRule

Summary: See https://discourse.phabricator-community.org/t/arc-diff-fails-on-incompatible-linter-rule-with-php7-3/2198.

Test Plan: Looked at the code carefully.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D19868