Modified VkNonDispatchableHandle so that it's a plain old data type.
By making sure VkNonDispatchableHandle is POD, it may benefit from
the same rules and register usage as the default object handles.

Bug b/129979580

Change-Id: I8aea419df8a6ee2ff95717424f3344e1c317fb28
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/32989Tested-by: Alexis Hétu <sugoi@google.com>
Presubmit-Ready: Alexis Hétu <sugoi@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>