Trigger bad charset lint warning only once per line

Summary:
Makes lots of noise:
{F22758}

Test Plan:
Linted file with several bad characters per line.
Linted file with one bad character per line.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3896