Disable default case checks on clang 18

Disable default case checks on clang 18

Recent updates in CI have upgraded clang to clang-18, which gripes when
it finds a switch statement without a default case.  We should add those
cases in, but since we have a lot of those, and CI is currently failing,
disable the check until we get them fixed up

Fixes #24739

Reviewed-by: Bernd Edlinger <bernd.edlinger@hotmail.de>
Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/24744)