c++: Allow defaulted comparison outside class.

c++: Allow defaulted comparison outside class.

Implementing P2085, another refinement to the operator<=> specification from
the Prague meeting.  It was deemed desirable to be able to have a non-inline
defaulted definition of a comparison operator just like you can with other
defaulted functions.

gcc/cp/ChangeLog:

	* method.c (early_check_defaulted_comparison): Allow defaulting
	comparison outside class.  Complain if non-member operator isn't a
	friend.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/spaceship-friend1.C: New test.
	* g++.dg/cpp2a/spaceship-err4.C: Adjust diagnostic.