Reject ALTER TABLE .. CLUSTER earlier for partitioned tables

Reject ALTER TABLE .. CLUSTER earlier for partitioned tables

ALTER TABLE .. CLUSTER ON and SET WITHOUT CLUSTER are not supported for
partitioned tables and already fail with a check happening when the
sub-command is executed, not when it is prepared.

This commit moves the relkind check for partitioned tables to happen
when the sub-command is prepared in ATSimplePermissions().  This matches
with the practice of the other sub-commands of ALTER TABLE, shaving one
translatable string.

mark_index_clustered() can be a bit simplified, switching one
elog(ERROR) to an assertion.  Note that mark_index_clustered() can also
be called through a CLUSTER command, but it cannot be reached for a
partitioned table, per the assertion based on the relkind in
cluster_rel(), and there is only one caller of rebuild_relation().

Author: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Reviewed-by: Zsolt Parragi <zsolt.parragi@percona.com>
Discussion: https://postgr.es/m/CAEoWx2kggo1N2kDH6OSfXHL_5gKg3DqQ0PdNuL4LH4XSTKJ3-g@mail.gmail.com