libstdc++: Simplify constraints on <=> for std::reference_wrapper

libstdc++: Simplify constraints on <=> for std::reference_wrapper

Instead of constraining these overloads in terms of synth-three-way we
can just check that the value_type is less-than-comparable, which is
what synth-three-way's constraints check.

The reason that I implemented these with constraints has now been filed
as LWG 4071, so add a comment about that too.

libstdc++-v3/ChangeLog:

	* include/bits/refwrap.h (operator<=>): Simplify constraints.