c++: more name lookup for non-dep rewritten cmp ops

c++: more name lookup for non-dep rewritten cmp ops

As a follow-up to r16-2448-g7590c14b53a762, this patch attempts to teach
build_min_non_dep_op_overload how to rebuild all rewritten comparison
operators, not just != -> == ones, so that we don't incorrectly repeat
the unqualified name lookup at instantiation time.

Note that changes how we mangle such comparison ops, in a way that is
overall more consistent with how we've been mangling non-rewritten
operators, see PR c++/121239.

gcc/cp/ChangeLog:

	* call.cc (build_new_op): If the selected candidate is
	rewritten, communicate the LOOKUP_REWRITTEN/REVERSED flags to
	the caller via the 'overload' out-parameter, and stop clearing
	'*overload' in that case.
	* tree.cc (build_min_non_dep_op_overload): Handle rebuilding all
	C++20 rewritten comparison operator expressions.

gcc/testsuite/ChangeLog:

	* g++.dg/lookup/operator-8.C: Remove XFAILs and properly
	suppress all -Wunused-result warnings.

Reviewed-by: Jason Merrill <jason@redhat.com>