Xcode (a popular code editor on Mac OS X) has no facility
to trim trailing whitespace automatically.
This adds a new lint severity "AUTOFIX" that's between
WARNING and ERROR. When running the linter, any lint message
whose severity is AUTOFIX will automatically be patched.
Furthermore, if all lint messages returned from the engine are
AUTOFIX, we'll automatically amend HEAD with the patch.
arc lint on files with and without trailing whitespace,
with and without UTF-8 contents to confirm those still error
Reviewers: epriestley, jungejason
Reviewed By: epriestley
Differential Revision: https://secure.phabricator.com/D2125