spirv: Handle the InputAttachmentIndex decoration

Reviewed-by: Dave Airlie <airlied@redhat.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>

src/compiler/spirv/vtn_private.h

@@ -279,6 +279,7 @@ struct vtn_variable {
 
    unsigned descriptor_set;
    unsigned binding;
+   unsigned input_attachment_index;
 
    nir_variable *var;
    nir_variable **members;