pgsql: Raise error on concurrent drop of partitioned index

Raise error on concurrent drop of partitioned index

We were already raising an error for DROP INDEX CONCURRENTLY on a
partitioned table, albeit a different and confusing one:
ERROR: DROP INDEX CONCURRENTLY must be first action in transaction

Change that to throw a more comprehensible error:
ERROR: cannot drop partitioned index \"%s\" concurrently

Michael Paquier authored the test case for indexes on temporary
partitioned tables.

Backpatch to 11, where indexes on partitioned tables were added.

Reported-by: Jan Mussler <jan(dot)mussler(at)zalando(dot)de>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/16594-d2956ca909585067@postgresql.org

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/7067ba1b4b905046819368e787809c5df0ebef04

Modified Files
--------------
doc/src/sgml/ref/drop_index.sgml | 2 ++
src/backend/commands/tablecmds.c | 11 +++++++++++
src/test/regress/expected/indexing.out | 20 ++++++++++++++++++++
src/test/regress/sql/indexing.sql | 13 +++++++++++++
4 files changed, 46 insertions(+)