Specify that atomic_ref member operations with workitem scope are UB.

[VerenaBeckham]

Addresses #1004.

[gmlueck]

I did a search of the spec for "memory_scope::work_item" to see where else we define behavior for this scope and found:

• https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#api:info-context-atomic-memory-scope-capabilities
• https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#api:info-device-atomic-memory-scope-capabilities

Both of these have wording like:

Returns: The set of memory scopes supported by atomic operations on this device. When a device returns a "wider" memory scope in this set, it must also return all "narrower" memory scopes. (See Section 3.8.3.2 for a definition of "wider" and "narrower" scopes.) At a minimum, each device must support memory_scope::work_item, memory_scope::sub_group, and memory_scope::work_group.

This has two problems:

• It says that devices must support memory_scope::work_item for atomic operations, which contradicts the clarification you are adding in this PR, and
• It says that the query must return all narrower scopes if it returns a wider scope. This won't be true if we simply remove memory_scope::work_item from the list.

[VerenaBeckham]

I did a search of the spec for "memory_scope::work_item" to see where else we define behavior for this scope and found:

* https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#api:info-context-atomic-memory-scope-capabilities

* https://registry.khronos.org/SYCL/specs/sycl-2020/html/sycl-2020.html#api:info-device-atomic-memory-scope-capabilities

Both of these have wording like:

Returns: The set of memory scopes supported by atomic operations on this device. When a device returns a "wider" memory scope in this set, it must also return all "narrower" memory scopes. (See Section 3.8.3.2 for a definition of "wider" and "narrower" scopes.) At a minimum, each device must support memory_scope::work_item, memory_scope::sub_group, and memory_scope::work_group.

This has two problems:

* It says that devices must support `memory_scope::work_item` for atomic operations, which contradicts the clarification you are adding in this PR, and

* It says that the query must return all narrower scopes if it returns a wider scope.  This won't be true if we simply remove `memory_scope::work_item` from the list.

You're right. Even if there is no harm in returning memory_scope::work_item from these queries, the way they are described ("scopes supported by atomic operations") implies that they should never return memory_scope::work_item.

I have committed a proposal, which defines a set of acceptable values from which to choose. This, I think, neatly circumvents the issue with the "wider"/"narrower" sentence. Perhaps the way the set is defined could be better worded, but let me know what you think of the approach.

[VerenaBeckham]

I also added some wording to the definition of the work_item scope. Perhaps not strictly necessary, but could be useful?

[gmlueck]

Looks good! Just a very small comment regarding grammar. Now that the list contains only two things, I think we don't want a comma here.

[TApplencourt]

• Wait for the CTS PR.
• Ready for Review