Simplify and speed up pg_dump's creation of parent-table links.

Simplify and speed up pg_dump's creation of parent-table links.

Instead of trying to optimize this by skipping creation of the
links for tables we don't plan to dump, just create them all in
bulk with a single scan over the pg_inherits data.  The previous
approach was more or less O(N^2) in the number of pg_inherits
entries, not to mention being way too complicated.

Also, don't create useless TableAttachInfo objects.
It's silly to create a TableAttachInfo object that we're not
going to dump, when we know perfectly well at creation time
that it won't be dumped.

Patch by me; thanks to Julien Rouhaud for review.

Discussion: https://postgr.es/m/1376149.1675268279@sss.pgh.pa.us