OpenCL: Use length modifier for warning on vector printf arguments

OpenCL: Use length modifier for warning on vector printf arguments

Re-enable format string warnings on printf.

The warnings are still incomplete. Apparently it is undefined to use a
vector specifier without a length modifier, which is not currently
warned on. Additionally, type warnings appear to not be working with
the hh modifier, and aren't warning on all of the special restrictions
from c99 printf.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@352540 91177308-0d34-0410-b5e6-96231b3b80d8