[D3D12] Fix `vertex_index` and `instance_index` built-in inputs for indirect draw calls

**Description**
`DrawIndexedIndirect.base_instance` is always 0 in the shader when using `draw_indexed_indirect` on DX12, regardless of the value provided. This works on Vulkan and Metal. Setting the `INDIRECT_FIRST_INSTANCE` flag doesn't help.

**Platform**
wgpu 0.12.0, Windows DX12

(Filed this issue by request from @kvark from a discussion in the matrix chat)