Add half limits

These are apparently only defined in OpenCL 1.2.

HALF_MAX, HALF_MIN and HALF_EPSILON are currently omitted. Clang does
not seem to support the ‘h’ suffix for half float constants even with
the cl_khr_fp16 extension enabled.

Reviewed-by: Tom Sellard <tom@stellard.net>
llvm-svn: 211579