Fix dumping role comments when using --no-role-passwords

Fix dumping role comments when using --no-role-passwords

Commit 9a83d56b38c added support for allowing pg_dumpall to dump
roles without including passwords, which accidentally made dumps
omit COMMENTs on roles.  This fixes it by using pg_authid to get
the comment.

Backpatch to all supported versions. Patch simultaneously written
independently by Álvaro and myself.

Author: Álvaro Herrera <alvherre@alvh.no-ip.org>
Author: Daniel Gustafsson <daniel@yesql.se>
Reported-by: Bartosz Chroł <bartosz.chrol@handen.pl>
Discussion: https://postgr.es/m/AS8P194MB1271CDA0ADCA7B75FCD8E767F7332@AS8P194MB1271.EURP194.PROD.OUTLOOK.COM
Discussion: https://postgr.es/m/CAEP4nAz9V4H41_4ESJd1Gf0v%3DdevkqO1%3Dpo91jUw-GJSx8Hxqg%40mail.gmail.com
Backpatch-through: v12