c++: More P2002 operator<=> refinements.

c++: More P2002 operator<=> refinements.

* Disallow && references.
* Allow empty unions.
* Improve diagnostics for a subobject comparison with
  non-comparison-category type.

gcc/cp/ChangeLog:

	* method.c (early_check_defaulted_comparison): Check for &&.
	(build_comparison_op): Allow empty union.  Diagnose non-category
	type.
	(common_comparison_type): Remove handling for non-category type.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp2a/spaceship-ref1.C: New test.
	* g++.dg/cpp2a/spaceship-synth-neg4.C: New test.
	* g++.dg/cpp2a/spaceship-union1.C: New test.