Clarify "atomic_ref" with private address space

[gmlueck]

The spec was not clear about this before. The OpenCL SPIR-V environment specification says that atomic operations on objects in the private address space produce undefined behavior. Therefore, it seems sensible to make the same clarification in SYCL.

[gmlueck]

The relevant text from the OpenCL SPIR-V Environment spec is here:

For all Atomic Instructions:

• Only 32-bit integer types are supported for the Result Type and/or type of Value.

• The Pointer operand must be a pointer to the Function, Workgroup, or CrossWorkgroup Storage Classes. Note that an Atomic Instruction on a pointer to the Function Storage Class is valid, but does not have defined behavior.

• For OpenCL environments that support and declare the GenericPointer capability, the Pointer operand may be a pointer to the Generic Storage Class, however behavior is still undefined if the Generic pointer represents a pointer to the Function Storage Class.

(https://registry.khronos.org/OpenCL/specs/3.0-unified/html/OpenCL_Env.html#validation-rules)

[bader]

The OpenCL SPIR-V environment specification says that atomic operations on objects in the address space produce undefined behavior.

the address space -> the private address space?