Reapply r360852 (by cem):
clang: Reject %n for attribute((format(freebsd_kprintf)))
A follow-up to r360849.
Reported by: imp
Reviewed by: emaste, imp
Differential Revision: https://reviews.freebsd.org/D24786
(cherry picked from commit ec280e9ec4b825ae73cdeccffeb8e72ce7dcc99d)