Fix tablespace handling for partitioned indexes

Fix tablespace handling for partitioned indexes

When creating partitioned indexes, the tablespace was not being saved
for the parent index. This meant that subsequently created partitions
would not use the right tablespace for their indexes.

ALTER INDEX SET TABLESPACE and ALTER INDEX ALL IN TABLESPACE raised
errors when tried; fix them too.  This requires bespoke code for
ATExecCmd() that applies to the special case when the tablespace move is
just a catalog change.

Discussion: https://postgr.es/m/20181102003138.uxpaca6qfxzskepi@alvherre.pgsql