The spec states that descriptor set layouts can be destroyed almost
at any time:

   "VkDescriptorSetLayout objects may be accessed by commands that
    operate on descriptor sets allocated using that layout, and those
    descriptor sets must not be updated with vkUpdateDescriptorSets
    after the descriptor set layout has been destroyed. Otherwise,
    descriptor set layouts can be destroyed any time they are not in
    use by an API command."

v2: allocate off the device allocator with DEVICE scope (Jason)

Fixes the following work-in-progress CTS tests:
dEQP-VK.api.descriptor_set.descriptor_set_layout_lifetime.graphics
dEQP-VK.api.descriptor_set.descriptor_set_layout_lifetime.compute

Suggested-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>