[libc++] Avoid -Wzero-as-null-pointer-constant in operator<=> (#79465)

Issue #43670 describes a situation where the following comparison will
issue a warning when -Wzero-as-null-pointer-constant is enabled:

    #include <compare>
    auto b = (1 <=> 2) < 0;

This code uses operator<(strong_ordering, Unspecified), which is
specified by the Standard to only work with a literal 0. In the library,
this is achieved by constructing Unspecified from a pointer, which works
but has the downside of triggering the warning.

This patch uses an alternative implementation where we require that the
operator is used exactly with an int of value 0 (known at compile-time),
however that value can technically be an expression like `1 - 1`, which
makes us a bit less strict than what's specified in the Standard.

Fixes #43670

0 files changed, 0 insertions, 0 deletions