[Spec change] Add shorthand aliases for address spaces

[sycl-issue-bot]

Please review whether KhronosGroup/SYCL-Docs#633 by @Pennycook requires any changes to the CTS.

If changes are required: Open a new PR addressing the changes and link it to this issue.
If no changes are required: Close this issue and proceed with the spec PR.

[Pennycook]

This change requires new tests, but they're very simple:

• Check that sycl::addrspace and sycl::access::address_space refer to the same type.
• Check that sycl::addrspace_global == sycl::addrspace::global_space.
• Check that sycl::addrspace_local == sycl::addrspace::local_space.
• Check that sycl::addrspace_private == sycl::addrspace::private_space.
• Check that sycl::addrspace_generic == sycl::addrspace::generic_space.