Improve lint handling of declare() special statement blocks

Summary: Fixes T6830. Don't require n_STATEMENT nested under n_DECLARE to be written using braces.

Test Plan: Added a test case.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T6830

Differential Revision: https://secure.phabricator.com/D11350