pgsql: Choose FK name correctly during partition attachment

Choose FK name correctly during partition attachment

During ALTER TABLE ATTACH PARTITION, if the name of a parent's foreign
key constraint is already used on the partition, the code tries to
choose another one before the FK attributes list has been populated,
so the resulting constraint name was "<relname>__fkey" instead of
"<relname>_<attrs>_fkey".  Repair, and add a test case.

Backpatch to 12.  In 11, the code to attach a partition was not smart
enough to cope with conflicting constraint names, so the problem doesn't
exist there.

Author: Jehan-Guillaume de Rorthais <jgdr@dalibo.com>
Discussion: https://postgr.es/m/20220901184156.738ebee5@karst

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/e7936f8b3e57046c0e178ccefa04ac7e6fbae79c

Modified Files
--------------
src/backend/commands/tablecmds.c          | 20 ++++++++++----------
src/test/regress/expected/constraints.out | 23 +++++++++++++++++++++++
src/test/regress/sql/constraints.sql      | 19 +++++++++++++++++++
3 files changed, 52 insertions(+), 10 deletions(-)