[libc++] Remove dependence on <ciso646> (#73271)

C++23 removed `<ciso646>` from the standard library. The header is used
in a few places in order to pull in implementation-specific and feature
test macros. The new way of doing that is `<version>`, which should be
supported by all supported implementations. This change replaces all
those uses of `<ciso646>` with `<version>`.