Fix CPP Lint severity

Summary: Currently all CPPLint issues are hard-coded to warning level, which prevents customising the severity in .arclint. Change to pick up the configured severity. Note that getLintMessageSeverity will call getDefaultMessageSeverity if nothing is configured for that error category.

Test Plan: Tested manually to confirm configured categories display with the correct severity and that non-configured ones return with the default severity (ERROR).

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D9682