Add a diagnostic triggered when a pipeline-constant is set for an override in the shader module that is not used by the entry point

[dneto0]

This is forked from #4624

During pipeline creation time, it is valid for a pipeline constant to be set for an override that exists in the shader but is not statically accessed by the entry point.
In the discussion from #4624 we agreed it would be nice to add a diagnostic, defaulting to off that would trigger in this case.
It's an extra guardrail that the developer can opt into.

To be determined:

• It certainly makes sense as a module-wide diagnostic directive.
• For a range diagnostic filter, where should it take affect or be allowed?
  • On the entry point?
  • On the declaration of the override? Currently range diagnostic filters are not specified to be allowed on a variable declaration.
• The name.