Fix postgres_fdw failure with whole-row Vars of type RECORD.

Commit 86dc90056 expects that FDWs can cope with whole-row Vars for
their tables, even if the Vars are marked with vartype RECORDOID.
Previously, whole-row Vars generated by the planner had vartype equal
to the relevant table's rowtype OID.  (The point behind this change is
to enable sharing of resjunk columns across inheritance child tables.)

It turns out that postgres_fdw fails to cope with this, though through
bad fortune none of its test cases exposed that.  Things mostly work,
but when we try to read back a value of such a Var, the expected
rowtype is not available to record_in().  Fortunately, it's not
difficult to hack up the tupdesc that controls this process to
substitute the foreign table's rowtype for RECORDOID.  Thus we can
solve the runtime problem while still sharing the resjunk column
with other tables.

Per report from Alexander Pyhalov.

Discussion: https://postgr.es/m/7817fb9ebd6661cdf9b67dec6e129a78@postgrespro.ru

contrib/postgres_fdw/expected/postgres_fdw.out

@@ -5531,6 +5531,31 @@ UPDATE ft2 AS target SET (c2) = (
         FROM ft2 AS src
         WHERE target.c1 = src.c1
 ) WHERE c1 > 1100;
+-- Test UPDATE involving a join that can be pushed down,
+-- but a SET clause that can't be
+EXPLAIN (VERBOSE, COSTS OFF)
+UPDATE ft2 d SET c2 = CASE WHEN random() >= 0 THEN d.c2 ELSE 0 END
+  FROM ft2 AS t WHERE d.c1 = t.c1 AND d.c1 > 1100;
+                                                                                                                                                                                       QUERY PLAN                                                                                                                                                                                        
+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ Update on public.ft2 d
+   Remote SQL: UPDATE "S 1"."T 1" SET c2 = $2 WHERE ctid = $1
+   ->  Foreign Scan
+         Output: CASE WHEN (random() >= '0'::double precision) THEN d.c2 ELSE 0 END, d.ctid, d.*, t.*
+         Relations: (public.ft2 d) INNER JOIN (public.ft2 t)
+         Remote SQL: SELECT r1.c2, r1.ctid, CASE WHEN (r1.*)::text IS NOT NULL THEN ROW(r1."C 1", r1.c2, r1.c3, r1.c4, r1.c5, r1.c6, r1.c7, r1.c8) END, CASE WHEN (r2.*)::text IS NOT NULL THEN ROW(r2."C 1", r2.c2, r2.c3, r2.c4, r2.c5, r2.c6, r2.c7, r2.c8) END FROM ("S 1"."T 1" r1 INNER JOIN "S 1"."T 1" r2 ON (((r1."C 1" = r2."C 1")) AND ((r1."C 1" > 1100)))) FOR UPDATE OF r1
+         ->  Nested Loop
+               Output: d.c2, d.ctid, d.*, t.*
+               ->  Foreign Scan on public.ft2 d
+                     Output: d.c2, d.ctid, d.*, d.c1
+                     Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8, ctid FROM "S 1"."T 1" WHERE (("C 1" > 1100)) ORDER BY "C 1" ASC NULLS LAST FOR UPDATE
+               ->  Foreign Scan on public.ft2 t
+                     Output: t.*, t.c1
+                     Remote SQL: SELECT "C 1", c2, c3, c4, c5, c6, c7, c8 FROM "S 1"."T 1" WHERE (($1::integer = "C 1"))
+(14 rows)
+
+UPDATE ft2 d SET c2 = CASE WHEN random() >= 0 THEN d.c2 ELSE 0 END
+  FROM ft2 AS t WHERE d.c1 = t.c1 AND d.c1 > 1100;
 -- Test UPDATE/DELETE with WHERE or JOIN/ON conditions containing
 -- user-defined operators/functions
 ALTER SERVER loopback OPTIONS (DROP extensions);