Improve undeclared variables linter rule

Summary: Currently ArcanistUndeclaredVariableXHPASTLinterRule does not properly handle anonymous closures.

Test Plan: Added test cases.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: johnny-bit, Korvin

Differential Revision: https://secure.phabricator.com/D13795