Fix the `ArcanistXHPASTLinter` single quote rule.

Summary: Previously, the `ArcanistXHPASTLinter` was suggesting that `"\1"` be written as `'\1'`, which is incorrect. Therefore, I ensured that all of the cases listed in the [[http://www.php.net/manual/en/language.types.string.php | PHP documentation]] were handled correctly.

Test Plan: `arc unit`

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9182