Allow more cases to pass the unsafe-use-of-new-enum-value restriction.

Up to now we've rejected cases like

BEGIN;
CREATE TYPE rainbow AS ENUM ();
ALTER TYPE rainbow ADD VALUE 'red';
-- use the value 'red', perhaps in a constraint or index
COMMIT;

The concern is that the uncommitted enum value 'red' might get into
an index and then break the index if we roll back the ALTER ADD.
If the ALTER is in the same transaction as the CREATE then it's really
perfectly safe, but we weren't taking the trouble to identify that.

pg_dump in binary-upgrade mode will emit enum definitions that look
like the above, which up to now didn't fall foul of the unsafe-usage
check because we processed each restore command as a separate
transaction.  However an upcoming patch proposes to bundle the restore
commands into large transactions to reduce XID consumption during
pg_upgrade, and that makes this behavior a problem.

To fix, remember the OIDs of enum types created in the current
transaction, and allow use of enum values that are added to one later
in the same transaction.  To do this fully correctly in the presence
of subtransactions, we'd have to track subtransaction nesting level of
the CREATE and do maintenance work at every subsequent subtransaction
exit.  That seems expensive, and we don't need it to satisfy pg_dump's
usage.  Hence, apply the additional optimization only when the CREATE
and ALTER are at outermost transaction level.

Patch by me, reviewed by Andrew Dunstan

Discussion: https://postgr.es/m/1548468.1711220438@sss.pgh.pa.us

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