Minor improvement to self member reference linter rule

Summary: Ref T7409. If :: is surrounded by whitespace tokens and the whitespace token contains a newline character, allow it.

Test Plan: Added test case.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7409

Differential Revision: https://secure.phabricator.com/D12391