pg_dump: include comments on valid not-null constraints, too

pg_dump: include comments on valid not-null constraints, too

We were missing collecting comments for not-null constraints that are
dumped inline with the table definition (i.e., valid ones), because they
aren't represented by a separately dumpable object.  Fix by creating
separate TocEntries for the comments.

Co-Authored-By: Jian He <jian.universality@gmail.com>
Co-Authored-By: Álvaro Herrera <alvherre@kurilemu.de>
Reported-By: Fujii Masao <masao.fujii@oss.nttdata.com>
Reviewed-By: Fujii Masao <masao.fujii@oss.nttdata.com>
Discussion: http://postgr.es/m/d50ff977-c728-4e9e-8488-fc2688e08754@oss.nttdata.com