Allow ViewportIndex & Layer to be used in VS/DS with extension

SPV_EXT_shader_viewport_index_layer enables using ViewportIndex
and Layer in vertex and tessellation shaders.

Also, as per the Vulkan spec:

> The ViewportIndex decoration must be used only within vertex,
> tessellation evaluation, geometry, and fragment shaders.

> In a vertex, tessellation evaluation, or geometry shader, any
> variable decorated with ViewportIndex must be declared using
> the Output storage class.

> In a fragment shader, any variable decorated with ViewportIndex
> must be declared using the Input storage class.

Similarly for Layer.