Harden xxx_is_visible() functions against concurrent object drops.

For the same reasons given in commit 403ac226d, adjust these
functions to not assume that checking SearchSysCacheExists can
guarantee success of a later fetch.

This follows the same internal API choices made in the earlier commit:
add a function XXXExt(oid, is_missing) and use that to eliminate
the need for a separate existence check.  The changes are very
straightforward, though tedious.  For the moment I just made the new
functions static in namespace.c, but we could export them if a need
emerges.

Per bug #18014 from Alexander Lakhin.  Given the lack of hard evidence
that there's a bug in non-debug builds, I'm content to fix this only
in HEAD.

Discussion: https://postgr.es/m/18014-28c81cb79d44295d@postgresql.org

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