layers:Minor refactor for clarity
layers:Minor refactor for clarity

Add comment and move VkSubpassDescription local variable assignment to
clarify what's going on.

KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers,KhronosGroup/Vulkan-LoaderAndValidationLayers