Don't allow partitioned index on foreign-table partitions

Creating indexes on foreign tables is already forbidden, but local
partitioned indexes (commit 8b08f7d4820f) forgot to check for them.  Add
a preliminary check to prevent wasting time.

Another school of thought says to allow the index to be created if it's
not a unique index; but it's possible to do better in the future (enable
indexing of foreign tables, somehow), so we avoid painting ourselves in
a corner by rejecting all cases, to avoid future grief (a.k.a. backward
incompatible changes).

Reported-by: Arseny Sher
Author: Amit Langote, Álvaro Herrera
Discussion: https://postgr.es/m/87sh71cakz.fsf@ars-thinkpad

src/test/regress/sql/foreign_data.sql

@@ -316,6 +316,12 @@ CREATE INDEX id_ft1_c2 ON ft1 (c2);                             -- ERROR
 SELECT * FROM ft1;                                              -- ERROR
 EXPLAIN SELECT * FROM ft1;                                      -- ERROR
 
+CREATE TABLE lt1 (a INT) PARTITION BY RANGE (a);
+CREATE FOREIGN TABLE ft_part1
+  PARTITION OF lt1 FOR VALUES FROM (0) TO (1000) SERVER s0;
+CREATE INDEX ON lt1 (a);                                        -- ERROR
+DROP TABLE lt1;
+
 -- ALTER FOREIGN TABLE
 COMMENT ON FOREIGN TABLE ft1 IS 'foreign table';
 COMMENT ON FOREIGN TABLE ft1 IS NULL;