pgsql: In pg_dumpall, don't skip role GRANTs with dangling grantor OIDs

In pg_dumpall, don't skip role GRANTs with dangling grantor OIDs.

In commits 29d75b25b et al, I made pg_dumpall's dumpRoleMembership
logic treat a dangling grantor OID the same as dangling role and
member OIDs: print a warning and skip emitting the GRANT. This wasn't
terribly well thought out; instead, we should handle the case by
emitting the GRANT without the GRANTED BY clause. When the source
database is pre-v16, such cases are somewhat expected because those
versions didn't prevent dropping the grantor role; so don't even
print a warning that we did this. (This change therefore restores
pg_dumpall's pre-v16 behavior for these cases.) The case is not
expected in >= v16, so then we do print a warning, but soldiering on
with no GRANTED BY clause still seems like a reasonable strategy.

Per complaint from Robert Haas that we were now dropping GRANTs
altogether in easily-reachable scenarios.

Reported-by: Robert Haas <robertmhaas(at)gmail(dot)com>
Author: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Discussion: https://postgr.es/m/CA+TgmoauoiW4ydDhdrseg+DD4Kwha=+TSZp18BrJeHKx3o1Fdw@mail.gmail.com
Backpatch-through: 16

Branch
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REL_16_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/a4649c50a8db58c19fa9a4f2ae12bef52b6c872b

Modified Files
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src/bin/pg_dump/pg_dumpall.c | 49 +++++++++++++++++++++++++++-----------------
1 file changed, 30 insertions(+), 19 deletions(-)