pgsql: Use generateClonedIndexStmt to propagate CREATE INDEX to partiti

Use generateClonedIndexStmt to propagate CREATE INDEX to partitions.

When instantiating an existing partitioned index for a new child
partition, we use generateClonedIndexStmt to build a suitable
IndexStmt to pass to DefineIndex.  However, when DefineIndex needs
to recurse to instantiate a newly created partitioned index on an
existing child partition, it was doing copyObject on the given
IndexStmt and then applying a bunch of ad-hoc fixups.  This has
a number of problems, primarily that it implies fresh lookups of
referenced objects such as opclasses and collations.  Since commit
2af07e2f7 caused DefineIndex to restrict search_path internally, those
lookups could fail or deliver different results than the original one.
We can avoid those problems and save a few dozen lines of code by
using generateClonedIndexStmt in this code path too.

Another thing this fixes is incorrect propagation of parent-index
comments to child indexes (because the copyObject approach copies
the idxcomment field while generateClonedIndexStmt doesn't).  I had
noticed this in connection with commit c01eb619a, but not run the
problem to ground.

I'm tempted to back-patch this further than v17, but the only thing
it's known to fix in older branches is the comment issue, which is
pretty minor and doesn't seem worth the risk of introducing new
issues in stable branches.  (If anyone does care about that,
clearing idxcomment in the copied IndexStmt would be a safer fix.)

Per bug #18637 from usamoi.  Back-patch to v17 where the search_path
change came in.

Discussion: https://postgr.es/m/18637-f51e314546e3ba2a@postgresql.org

Branch
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REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/fee8cb9473462e023dcf0b41212ca3890ddc28d6

Modified Files
--------------
contrib/seg/Makefile                      |  2 +-
contrib/seg/expected/partition.out        | 54 +++++++++++++++++++++++++++++
contrib/seg/meson.build                   |  1 +
contrib/seg/sql/partition.sql             | 36 ++++++++++++++++++++
src/backend/commands/indexcmds.c          | 56 ++++++-------------------------
src/test/regress/expected/alter_table.out |  5 ++-
src/test/regress/sql/alter_table.sql      |  2 --
7 files changed, 105 insertions(+), 51 deletions(-)