[OpenCL] Adjust diagnostic for subgroup support.

OpenCL C 3.0 __opencl_c_subgroups feature is slightly different
then other equivalent features and extensions (fp64 and 3d image writes):
OpenCL C 3.0 device can support the extension but not the feature.
cl_khr_subgroups requires subgroup independent forward progress.

This patch adjusts the check which is used when translating language
builtins to check either the extension or feature is supported.

Reviewed By: Anastasia

Differential Revision: https://reviews.llvm.org/D118999