spirv: Support EXT_mesh_shader indices and mark them per-primitive.

spirv: Support EXT_mesh_shader indices and mark them per-primitive.

They are not defined as per-primitive in the EXT, but they behave
like per-primitive outputs so it's easier to treat them like that.
They may still require special treatment in the backend in order to
control where and how they are stored.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18366>