Don't run disabled lint rules

Summary:
We always generate all messages and then filter them out based on minimum severity.
It's lots of useless work, especially in commit hook mode where we are interested only in errors.

Test Plan:
  $ arc lint --cache 0 --severity error ArcanistXHPASTLinter.php
  0.406 s before, 0.074 after

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4963