c++: non-dep cmp op rewritten from <=> returning int [PR121779]

c++: non-dep cmp op rewritten from <=> returning int [PR121779]

Apparently an explicitly defined operator<=> isn't required to return
std::foo_ordering, so build_min_non_dep_op_overload needs to be able
to rebuild forms of (x <=> y) @ 0 where the @ resolved to a built-in and
in turn isn't expressed as a function call.

	PR c++/121779

gcc/cp/ChangeLog:

	* tree.cc (build_min_non_dep_op_overload): Handle comparison
	operator expressions rewritten from a <=> that returns a
	non-class type.

gcc/testsuite/ChangeLog:

	* g++.dg/lookup/operator-8.C: Remove outdated comment about
	this test failing.
	* g++.dg/lookup/operator-8a.C: New test.

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