SPIR-V: Required Scope for WorkgroupMemory Atomics?

I found this issue addressing a review comment in #84.

Currently, in the SPIR-V environment specification section [7.2.8,](https://www.khronos.org/registry/OpenCL/specs/2.2/html/OpenCL_Env.html#opencl_extensions) when describing behavior when the extensions `cl_khr_in64_base_atomics` and `cl_khr_int64_extended_atomics` are supported, there is the line:

> When the **WorkgroupMemory** *Memory Semantic* is used the *Scope* must be **Workgroup**.

However, we don't include any similar validation rule for non-64-bit atomics. Should we?

Regardless, I think this should be described as a common validation rule, and not tied specifically to the 64-bit atomics extensions.

For reference, the [OpenCL C spec ](https://www.khronos.org/registry/OpenCL/specs/2.2/html/OpenCL_C.html#atomic-functions) says:

> Atomic operations to `local` memory only guarantee memory ordering in the work-group not across work-groups and therefore ignore the `memory_scope` value.

Start by reviewing SPIR-V environment specification section 7.2.8 and the referenced OpenCL C atomic-functions section, then compare the existing 64-bit atomic validation rule with non-64-bit atomics. Done means the scope requirement is resolved and, if applicable, documented as a common validation rule rather than only under the 64-bit extensions.

Written by the indexing model from the issue text.