Consider magic scalars to be constant strings

Summary: Consider magic scalars such as __DIR__ to be constant strings. I am using this to determine if a path that is imported with require_once is autoloadable, see D12271 for some additional context.

Test Plan: Tested externally with a custom linter.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12270