`@builtin(vertex_instance)` and `@builtin(instance_index)` not supported for `multi_draw_indirect_count` on DX12

[tychedelia]

#7535 added support for these builtins for regular indirect drawing on dx12, but they don't work when using the count variant, which can make using mdi count difficult in practice.

Ideally these would be implemented, but I understand that may be difficult. Alternatively, there should be a downlevel flag indicating whether these are available.

Encountered in Bevy bevyengine/bevy#19868