See PHI1634.

• In D20986, the "LintRenderer" classes swapped to a modern modular pattern.
• This invokes all of their constructors as a side effect.
• The ArcanistCheckstyleXMLLintRenderer linter is written in an un-modern way and causes significant side effects during construction, notably failing if the XML extension which provides XMLWriter is not installed.

Instead:

• ArcanistCheckstyleXMLLintRenderer should not instantiate XMLWriter on construction.
• If the xml extension is not available, ArcanistCheckstyleXMLLintRenderer should fail more clearly/gracefully.