Move the getFunctionCalls method to ArcanistBaseXHPASTLinter

Summary: Move the getFunctionCalls method to the ArcanistBaseXHPASTLinter so that it can be used by subclasses.

Test Plan: arc unit

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D12472