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