Non-legacy multi_ptr and access::address_space::constant_space

The specification does not explicitly state that `access::address_space::constant_space` is unsupported by non-legacy `multi_ptr`, but it does specify that:

> This legacy class supports the deprecated address_space::constant_space address space, which can be used to represent a pointer to [constant memory](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#constant-memory). Pointers to [constant memory](https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#constant-memory) have an implementation-defined address space, and each implementation can define whether it is legal to assign such a pointer to a generic address pointer.

Additionally it disallows `access::address_space::constant_space` in the RHS of the assignment operator overloads of non-legacy `multi_ptr` and `constant_ptr` is defined as a legacy `multi_ptr`. Is the intention that non-legacy `multi_ptr` shouldn't support `access::address_space::constant_space`?

Review the SYCL 2020 specification text on non-legacy multi_ptr, access::address_space::constant_space, and the assignment operator overloads, then compare it with the statement about legacy multi_ptr and constant memory. Determine whether constant_space is intended to be unsupported for non-legacy multi_ptr and constant_ptr, and clarify the specification accordingly.

Written by the indexing model from the issue text.