Add sycl_khr_work_item_queries extension

[Pennycook]

This extension allows developers to access instances of the sycl::nd_item, sycl::group and sycl::sub_group classes without having to explicitly pass them as arguments to each function used on the device.

[keryell]

Thanks!

[tomdeakin]

Awaiting CTS

[jbrodman]

So this is a bit late, but could we consider collapsing some of this down? the this_work_item:: namespace doesn't seem to be doing much more than adding char count. Something maybe like sycl::this_work_item() seems just as intuitive.

[Pennycook]

So this is a bit late, but could we consider collapsing some of this down? the this_work_item:: namespace doesn't seem to be doing much more than adding char count. Something maybe like sycl::this_work_item() seems just as intuitive.

After some offline discussion, the names that people seem to like most are:

namespace sycl {
namespace khr {

template <int Dimensions>
nd_item<Dimensions> this_nd_item();

template <int Dimensions>
group<Dimensions> this_group();

sub_group this_sub_group();

}
}

...which would naturally extend to include this_work_item(), this_work_group() and this_root_group() from other KHRs.