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