Add a validation warning when both sampleShading and pipeline/primitive/attachmentShadingRate are enabled for a VkPipeline

According to the proposal doc for VK_KHR_fragment_shading_rate :

Note though that enabling sample shading will effectively disable the fragment shading rate.

No validation errors or warnings are raised when a developer enables both simultaneously,
resulting in the Vulkan implementation silently disabling the fragment shading rate without any indication as to why this may have occurred.

It took awhile for me to determine the cause when I did this-
enabled sample shading, noticed fragment shading rate was disabled only days later,
and by that time, I had made so many other changes that I really had no idea what had caused VRS to shut off.

To quote Spencer on the matter: "Seems like a foot gun", and speaking from personal experience,
I'm inclined to agree =v=

Start with the VK_KHR_fragment_shading_rate proposal linked in the issue and trace the validation handling for sample shading and pipeline/primitive/attachmentShadingRate. Done means a validation warning is raised when both are enabled, explaining that sample shading disables the fragment shading rate; add or update coverage for this combination if the relevant test entry point is found.