PR44723: Trigger return type deduction for operator<=>s whose return types are needed to compute the return type of a defaulted operator<=>.
This raises the question of what to do if return type deduction fails.
The standard doesn't say, and implementations vary, so for now reject
that case eagerly to keep our options open.
(cherry picked from commit 42d4a55f227a1cc78ab8071062d869abe88655d9)