Improve the "invalid parent scope" linter rule

Summary: This linter rule was introduced in D12420, but fails to handle a specific edge case.

Test Plan: Added test case.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12853