Minor improvements to the ArcanistLintCheckstyleXMLRenderer class.
Summary: Ref T4948. Move the startDocument code to a renderPreamble function so that, at least theoretically, the renderer can be reused. Otherwise, the only way to reuse the renderer would be to construct a new instance.
Test Plan: Ran arc lint --output xml and verified that the output looked reasonable.
Reviewers: Blessed Reviewers, epriestley
Reviewed By: Blessed Reviewers, epriestley
Subscribers: epriestley, Korvin
Maniphest Tasks: T4948
Differential Revision: https://secure.phabricator.com/D9108