[OpenCL] Add intel_reqd_sub_group_size attribute support

Summary:
Add intel_reqd_sub_group_size attribute support as intel extension  cl_intel_required_subgroup_size from
https://www.khronos.org/registry/OpenCL/extensions/intel/cl_intel_required_subgroup_size.txt

Reviewers: Anastasia, bader, hfinkel, pxli168

Reviewed By: Anastasia, bader, pxli168

Subscribers: cfe-commits, yaxunl

Differential Revision: https://reviews.llvm.org/D30805

llvm-svn: 302125