Improve the "inline HTML" linter rule
Summary: Improve ArcanistInlineHTMLXHPASTLinterRule such that it doesn't raise duplicate warnings. Also be a bit more lax with whitespace.
Test Plan: Unit tests now pass.
Reviewers: #blessed_reviewers, epriestley
Reviewed By: #blessed_reviewers, epriestley
Subscribers: Korvin
Differential Revision: https://secure.phabricator.com/D13896