[SYCL] Align item with SYCL 2020 (#22907)
Closes #22870.
Align `sycl::item` with the SYCL 2020 specification:
- Add `noexcept` to the `item` query and conversion functions.
- Implement the equality operators as hidden friends.
- Add compile-time tests covering the exception specifications and
hidden-friend lookup.
