pgsql: Fix dumping of comments on invalid constraints on domains

Fix dumping of comments on invalid constraints on domains

We skip dumping constraints together with domains if they are invalid
('separate') so that they appear after data -- but their comments were
dumped together with the domain definition, which in effect leads to the
comment being dumped when the constraint does not yet exist. Delay
them in the same way.

Oversight in 7eca575d1c28; backpatch all the way back.

Author: jian he <jian(dot)universality(at)gmail(dot)com>
Discussion: https://postgr.es/m/CACJufxF_C2pe6J_+nPr6C5jf5rQnbYP8XOKr4HM8yHZtp2aQqQ@mail.gmail.com

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/cef998ef8314ea88db15b66f21ac000c9d0c248c

Modified Files
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src/bin/pg_dump/pg_dump.c | 23 ++++++++++++++++++++++-
src/test/regress/expected/constraints.out | 4 ++++
src/test/regress/sql/constraints.sql | 6 ++++++
3 files changed, 32 insertions(+), 1 deletion(-)