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
aa08843878