Improve strictness around setLine() types in ArcanistLintMessage

Summary: Fixes T8921. Harbormaster is strict about types it accepts, but ArcanistLintMessage is more liberal. Push the strictness barrier down to the linter level, while maintaining reasonable flexibility in the API.

Test Plan: arc unit --everything

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T8921

Differential Revision: https://secure.phabricator.com/D13695