Inherit member ids from base for dyn disc type

This adds the the member ids from the base type to the member ids of a
struct when dynamically constructing a struct type from a TypeObject.

Without this step, the inherited fields end up getting member id -1
somewhere in the of the deserializer and cause the inherited fields to
not be found during deserialization.  Because of this, none of the
inherited fields will be deserialized and a must-understand field (which
includes key fields) among them causes a deserialization error.

Signed-off-by: Erik Boasson <eb@ilities.com>