Render chevrons and carets in lint output for messages without "original" text

Summary:
If a message has only "line", or "line" and "char", we don't point out the location in the context block.

When a message includes "line", show chevrons on the line.
When a message includes "line" and "char", show chevrons on the line and a caret on the next line.

Test Plan: Ran regex linters to capture line, line+char, and normal linters to capture everything. Output appeared sane.

Reviewers: csilvers, vrana, btrahan

Reviewed By: csilvers

CC: aran

Differential Revision: https://secure.phabricator.com/D2642