Use a blacklist to distinguish original from add-on enum values.

Commit 15bc038f9 allowed ALTER TYPE ADD VALUE to be executed inside
transaction blocks, by disallowing the use of the added value later
in the same transaction, except under limited circumstances.  However,
the test for "limited circumstances" was heuristic and could reject
references to enum values that were created during CREATE TYPE AS ENUM,
not just later.  This breaks the use-case of restoring pg_dump scripts
in a single transaction, as reported in bug #14825 from Balazs Szilfai.

We can improve this by keeping a "blacklist" table of enum value OIDs
created by ALTER TYPE ADD VALUE during the current transaction.  Any
visible-but-uncommitted value whose OID is not in the blacklist must
have been created by CREATE TYPE AS ENUM, and can be used safely
because it could not have a lifespan shorter than its parent enum type.

This change also removes the restriction that a renamed enum value
can't be used before being committed (unless it was on the blacklist).

Andrew Dunstan, with cosmetic improvements by me.
Back-patch to v10.

Discussion: https://postgr.es/m/20170922185904.1448.16585@wrigleys.postgresql.org

doc/src/sgml/ref/alter_type.sgml

@@ -294,8 +294,6 @@ ALTER TYPE <replaceable class="PARAMETER">name</replaceable> RENAME VALUE <repla
    an enum type) is executed inside a transaction block, the new value cannot
    be used until after the transaction has been committed, except in the case
    that the enum type itself was created earlier in the same transaction.
-   Likewise, when a pre-existing enum value is renamed, the transaction must
-   be committed before the renamed value can be used.
   </para>
 
   <para>