Add support for analyzing FreeBSD kernel printf extensions.

This adds a new __freebsd_kprintf__ format string type, which enables
checking when used in __attribute__((format(...))) attributes.  It can
check the FreeBSD kernel specific %b, %D, %r and %y specifiers, using
existing diagnostic messages.  Also adds test cases for all these
specifiers.

Differential Revision: http://reviews.llvm.org/D7154

llvm-svn: 229921