`@name` on top-level type silently does nothing

[SebastienGllmt]

Right now if you do something like

out_ref = _CDDL_CODEGEN_EXTERN_TYPE_ ; @name Foo

The @name notation will silently do nothing (it only works for giving names to things inside structs). We don't need to support this, but if we don't we should probably throw an error instead of silently doing nothing