c++: fix 32-bit spaceship failures [PR110185]

Various spaceship tests failed after r14-1624.  This turned out to be
because the comparison category classes return in memory on 32-bit targets,
and the synthesized operator<=> looks something like

if (auto v = a.x <=> b.x, v == 0); else return v;
if (auto v = a.y <=> b.y, v == 0); else return v;
etc.

so check_return_expr was trying to do NRVO for all the 'v' variables, and
now on subsequent returns we check to see if the previous NRV is still in
scope.  But the NRVs didn't have names, so looking up name bindings crashed.
Fixed both by giving 'v' a name so we can NRVO the first one, and fixing the
test to give up if the old NRV has no name.

	PR c++/110185
	PR c++/58487

gcc/cp/ChangeLog:

	* method.cc (build_comparison_op): Give retval a name.
	* typeck.cc (check_return_expr): Fix for nameless variables.