qapi-types: Drop unnedeed ._fwdefn

Previously, the generated code in qapi-types.c initialized all
enum lookup tables first, prior to any other definitions.  But
there are no topological sorting requirements that mandate this
layout, so we can drop the QAPISchemaGenTypeVisitor._fwdefn
field and just generate all definitions in visitation order.

The generated code shows some churn due to reordering, but it
is still fairly straightforward to follow (all the deletions
occur in one hunk, and all the deleted lines are re-inserted
in the same order later in the same files, just spread across
multiple insertion points).

Suggested-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <1449033659-25497-6-git-send-email-eblake@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
