pgsql: Harden xxx_is_visible() functions against concurrent object drop

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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/fcdd6689d09c3950efa8a79db872f507069adb92

Modified Files
--------------
src/backend/catalog/namespace.c | 387 +++++++++++++++++++++++++++++++++++-----
src/include/catalog/namespace.h | 2 +-
2 files changed, 343 insertions(+), 46 deletions(-)