c++: ambiguous member lookup for rewritten cands [PR113529]

c++: ambiguous member lookup for rewritten cands [PR113529]

Here we handle the operator expression u < v inconsistently: in a SFINAE
context we accept it, and in a non-SFINAE context we reject it with

  error: request for member 'operator<=>' is ambiguous

as per [class.member.lookup]/6.  This inconsistency is ultimately
because we neglect to propagate error_mark_node after recursing in
add_operator_candidates, fixed like so.

	PR c++/113529

gcc/cp/ChangeLog:

	* call.cc (add_operator_candidates): Propagate error_mark_node
	result after recursing to find rewritten candidates.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/spaceship-sfinae3.C: New test.

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