pgsql: Teach pg_dump to dump comments on RLS policy objects.

Teach pg_dump to dump comments on RLS policy objects.

This was unaccountably omitted in the original RLS patch.
The SQL syntax is basically the same as for comments on triggers,
so crib code from dumpTrigger().

Per report from Marc Munro.  Back-patch to all supported branches.

Discussion: https://postgr.es/m/1581889298.18009.15.camel@bloodnok.com

Branch
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REL9_6_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/3380b99312c3ae80fdbb4e1745677f759b8f1ee1

Modified Files
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src/bin/pg_dump/pg_dump.c | 15 +++++++++++++++
1 file changed, 15 insertions(+)