Fix custom severity rules not being applied

Summary: It seems that severity.rules does not override the default linter severities set by the ArcanistLinter itself. This seems incorrect and counter-intuitive.

Test Plan: Created a "severity.rules": { "(.*)": "disabled" }" configuration and verified that it was being applied.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D10475