Improve ArcanistXHPASTLinter::LINT_TOSTRING_EXCEPTION

Summary:
The ArcanistXHPASTLinter::LINT_TOSTRING_EXCEPTION linter rule was introduced in D12854, but fails for the following:

• Interfaces which declare the __toString() method.
• Classes which mark the __toString() method as abstract.

Test Plan: Added test cases.

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12892