Fix pg_dump's handling of extension-member casts and languages.

pg_dump has some heuristic rules for whether to dump casts and procedural
languages, since it's not all that easy to distinguish built-in ones from
user-defined ones.  However, we should not apply those rules to objects
that belong to an extension, but just use the perfectly well-defined rules
for what to do with extension member objects.  Otherwise we might
mistakenly lose extension member objects during a binary upgrade (which is
the only time that we'd want to dump extension members).