Fix pg_dump's failure to honor dependencies of SQL functions.

Fix pg_dump's failure to honor dependencies of SQL functions.

A new-style SQL function can contain a parse-time dependency
on a unique index, much as views and matviews can (such cases
arise from GROUP BY and ON CONFLICT clauses, for example).
To dump and restore such a function successfully, pg_dump must
postpone the function until after the unique index is created,
which will happen in the post-data part of the dump.  Therefore
we have to remove the normal constraint that functions are
dumped in pre-data.  Add code similar to the existing logic
that handles this for matviews.  I added test cases for both
as well, since code coverage tests showed that we weren't
testing the matview logic.

Per report from Sami Imseih.  Back-patch to v14 where
new-style SQL functions came in.

Discussion: https://postgr.es/m/2C1933AB-C2F8-499B-9D18-4AC1882256A0@amazon.com