Can "item" and "nd_item" be shared across work-items?

This question recently came up from our implementation team. What is the precise semantics of the member functions on `item` and `nd_item`? Do they return the index of the *calling* work-item or do they return the index of the work-item that created the object?

The question is relevant only in the weird case where an `item` or `nd_item` object from one work-item is copied into SLM and then referenced by another work-item. This could be done, for example, by using placement-new to copy the `item` or `nd_item` object.

Note that `id` may have a different semantic because it is user-constructible. The semantics of `id` seem clear. The member functions return the information that was used to construct the `id` object. Therefore, if work-item _A_ copies its `id` object to SLM and then another work-item _B_ calls its member functions, I presume work-item _B_ would get the index of work-item _A_.

Start by reviewing the SYCL specification text that defines the member functions of item, nd_item, and id, along with the described copying into SLM. Done means the specification records precise semantics for calls through copied item and nd_item objects and distinguishes them from user-constructed id objects.

Written by the indexing model from the issue text.