qapi: Create simple union type member earlier

qapi: Create simple union type member earlier

For simple unions, we were creating the implicit 'type' tag
member during the QAPISchemaObjectTypeVariants constructor.
This is different from every other implicit QAPISchemaEntity
object, which get created by QAPISchema methods.  Hoist the
creation to the caller (renaming _make_tag_enum() to
_make_implicit_tag()), and pass the entity rather than the
string name, so that we have the nice property that no
entities are created as a side effect within a different
entity.  A later patch will then have an easier time of
associating location info with each entity creation.

No change to generated code.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1444710158-8723-10-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>