libstdc++: Make __cmp_cat::__unseq constructor consteval

libstdc++: Make __cmp_cat::__unseq constructor consteval

This constructor should only ever be used with a literal 0 as the
argument, so we can make it consteval. This has the nice advantage that
it is expanded immediately in the front end, and so GDB will never step
into the __cmp_cat::__unseq::__unseq(__unseq*) constructor that is
uninteresting and probably confusing to users.

libstdc++-v3/ChangeLog:

	* libsupc++/compare (__cmp_cat::__unseq): Make ctor consteval.
	* testsuite/18_support/comparisons/categories/zero_neg.cc: Prune
	excess errors caused by invalid consteval calls.