Creating indexes on foreign tables is already forbidden, but local
partitioned indexes (commit 8b08f7d4) 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