pgsql: Rename C23 keyword

Rename C23 keyword

constexpr is a keyword in C23.  Rename a conflicting identifier for
future-proofing.

Reviewed-by: Robert Haas <robertmhaas@gmail.com>
Discussion: https://www.postgresql.org/message-id/flat/08abc832-1384-4aca-a535-1a79765b565e%40eisentraut.org
Discussion: https://www.postgresql.org/message-id/flat/87o72eo9iu.fsf%40gentoo.org

Branch
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REL_17_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ad89c8bda1f096a8592f1c27abec1e34e5f10408

Modified Files
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src/backend/optimizer/util/predtest.c | 20 ++++++++++----------
1 file changed, 10 insertions(+), 10 deletions(-)