Sema: Warn on sizeof on binary ops on decayed arrays.

The array will decay into a pointer, creating an unexpected result.
sizeof(array + int) is an easy to make typo for sizeof(array) + int.

This was motivated by a NetBSD security bug, used sizeof(key - r) instead of
sizeof(key) - r, reducing entropy in a random number generator.
http://cvsweb.netbsd.org/bsdweb.cgi/src/sys/kern/subr_cprng.c.diff?r1=1.14&r2=1.15&only_with_tag=MAIN&f=h

Differential Revision: http://llvm-reviews.chandlerc.com/D571