Move the getSuperGlobalNames method to ArcanistBaseXHPASTLinter

Summary: Move the getSuperGlobalNames method to ArcanistBaseXHPASTLinter so that it can be used within subclasses.

Test Plan: arc unit

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley, Blessed Reviewers

Subscribers: virendra20888, Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12473