SPIR-V: Validation of modules using subgroups in 2.0 environment

There doesn't seem to be a way to specify in a SPIR-V module whether cl_khr_subgroups is supported which makes validating a module making use of **Subgroup** _Scope_ in an OpenCL 2.0 environment undecidable. If cl_khr_subgroups is supported the module is valid, otherwise, it is not.

Do we need to create a new capability?

Start with the issue's description of SPIR-V modules, Subgroup Scope, OpenCL 2.0, and cl_khr_subgroups; no source files or tests are named. Investigate whether the validation gap requires a new capability, and consider the issue complete when the specification direction is decided and documented.

Written by the indexing model from the issue text.