[OpenCL] Add cl_khr_subgroup_ballot to TableGen BIFs

Add the builtin functions brought by the cl_khr_subgroup_ballot
extension to `-fdeclare-opencl-builtins`.

Also add placeholder comments for the other Extended Subgroup
Functions from the OpenCL Extension Specification.

Add a comment clarifying the scope of the test.

Differential Revision: https://reviews.llvm.org/D95523