vk::DescriptorSet has a header which has a pointer to the descriptor set
layout, which is needed for vkUpdateDescriptorSets. This information
should not be available to shaders during execution.

Bug: b/152227757
Change-Id: I088828a28ab4ed75ae84fb3f103455f70a8bc051
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/43968
Presubmit-Ready: Nicolas Capens <nicolascapens@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>