SPIR-V environment: Validation rules rule out linkable modules with unknown WorkgroupSize

The following rule:

> VUID-WorkgroupSize-WorkgroupSize-04426
> The variable decorated with WorkgroupSize must be a specialization constant or a constant

prevents creating a shader module that refers to an indeterminate workgroup size. It forces the use of (spec) constants which is problematic since you cannot encode a constant with no value, and specifying a bogus sentinel value like (0,0,0) is also ruled out. This runs into conflict with what the SPIR-V spec suggests: (emphasis mine)

> Workgroup: The set of invocations partitioned in some execution models (e.g. GLCompute, Kernel) as a
workgroup. Its size is defined statically by either the WorkgroupSize built-in or the LocalSize or
LocalSizeId Execution Modes, or can be _queried_ via the WorkgroupSize built-in.

that we can _query_ the workgroup size, without defining it.

For clarity this issue concerns shader modules using the `Linkage` capability, _not_ modules fit for direct consumption by a Vulkan application.

Start by reviewing VUID-WorkgroupSize-WorkgroupSize-04426 alongside the cited SPIR-V Workgroup definition and the Linkage capability context. Determine how validation should handle linkable modules whose workgroup size is queried rather than defined; done means resolving the conflict without weakening validation for modules intended for direct Vulkan consumption.

Written by the indexing model from the issue text.