Make "line" and "char" strictly optional in ArcanistLintMessage

Summary: See PHI136. These are already optional on the server side in HarbormasterBuildLintMessage, and effectively mean "file-level issue", which is a bit niche but not unreasonable.

Test Plan: Checked that HarbormasterBuildLintMessage doesn't care if these keys exist, created this revision.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18711