XMI12 — EmbeddedObject is misused

Assuming that issue 3822 is resolved as Dave Frankel suggests, we have a
problem with the way that the EmbeddedObject production is used in other
productions; e.g. MvAttributeContents and SvAttributeContents. In these
cases (and possibly others), EmbeddedObject may generate embedded XML
for
a Class instance that needs to be referenced (via an xmi.idref)
elsewhere
in the document. But the EmbeddedObject production does not include the
necessary "xmi.id" attribute to make this work.

The easiest solution is to get rid of the EmbeddedObject production
altogether, and replace all uses with ObjectAsElement.