sycl::is_group is missing

[Pennycook]

Describe the bug
sycl::is_group is missing, and sycl::is_group_v is defined directly in terms of some structs in detail:

llvm/sycl/include/sycl/detail/type_traits.hpp

Lines 67 to 69 in 90e0e65

	template <class T>
	inline constexpr bool is_group_v =
	detail::is_group<T>::value || detail::is_sub_group<T>::value;

To Reproduce

#include <sycl/sycl.hpp>

static_assert(sycl::is_group<sycl::group<1>>::value);
static_assert(sycl::is_group<sycl::sub_group>::value);

The above fails to compile, producing errors like:

test.cpp:3:15: error: no template named 'is_group' in namespace 'sycl'; did you mean 'sycl::detail::is_group'?
static_assert(sycl::is_group<sycl::group<1>>::value);
              ^~~~~~~~~~~~~~
              sycl::detail::is_group

Environment:

• OS: Linux
• Target device and vendor: N/A
• DPC++ version: 90e0e65
• Dependencies version: N/A

Additional context
We should match the behavior described in Section 4.17.3.1 of the SYCL 2020 specification; in addition to defining sycl::is_group, we should define sycl::is_group_v in terms of that trait.

I'm trying to implement some new group types, and I expected to be able to just define a new specialization of sycl::is_group for those classes. I'm afraid to update the definition of sycl::is_group_v myself because I'm not sure what will break.