c++: Fix defaulted <=> fallback to < and == [PR96299]

c++: Fix defaulted <=> fallback to < and == [PR96299]

I thought I had implemented P1186R3, but apparently I didn't read it closely
enough to understand the point of the paper, namely that for a defaulted
operator<=>, if a member type doesn't have a viable operator<=>, we will use
its operator< and operator== if the defaulted operator has an specific
comparison category as its return type; the compiler can't guess if it
should be strong_ordering or something else, but the user can make that
choice explicit.

The libstdc++ test change was necessary because of the change in
genericize_spaceship from op0 > op1 to op1 < op0; this should be equivalent,
but isn't because of PR88173.

gcc/cp/ChangeLog:

	PR c++/96299
	* cp-tree.h (build_new_op): Add overload that omits some parms.
	(genericize_spaceship): Add location_t parm.
	* constexpr.c (cxx_eval_binary_expression): Pass it.
	* cp-gimplify.c (genericize_spaceship): Pass it.
	* method.c (genericize_spaceship): Handle class-type arguments.
	(build_comparison_op): Fall back to op</== when appropriate.

gcc/testsuite/ChangeLog:

	PR c++/96299
	* g++.dg/cpp2a/spaceship-synth-neg2.C: Move error.
	* g++.dg/cpp2a/spaceship-p1186.C: New test.

libstdc++-v3/ChangeLog:

	PR c++/96299
	* testsuite/18_support/comparisons/algorithms/partial_order.cc:
	One more line needs to use VERIFY instead of static_assert.