Add hardening to catch invalid TIDs in indexes.

Add hardening to the heapam index tuple deletion path to catch TIDs in
index pages that point to a heap item that index tuples should never
point to.  The corruption we're trying to catch here is particularly
tricky to detect, since it typically involves "extra" (corrupt) index
tuples, as opposed to the absence of required index tuples in the index.

For example, a heap TID from an index page that turns out to point to an
LP_UNUSED item in the heap page has a good chance of being caught by one
of the new checks.  There is a decent chance that the recently fixed
parallel VACUUM bug (see commit 9bacec15) would have been caught had
that particular check been in place for Postgres 14.  No backpatch of
this extra hardening for now, though.

Author: Peter Geoghegan <pg@bowt.ie>
Reviewed-By: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/CAH2-Wzk-4_raTzawWGaiqNvkpwDXxv3y1AQhQyUeHfkU=tFCeA@mail.gmail.com

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