libstdc++: Implement spaceship for std::pair (P1614R2)

libstdc++: Implement spaceship for std::pair (P1614R2)

This defines operator<=> as a non-member function template and does not
alter operator==. This contradicts the changes made by P1614R2, which
specify both as hidden friends, but that specification of operator<=> is
broken and the subject of a soon-to-be-published LWG issue.

	* include/bits/stl_pair.h [__cpp_lib_three_way_comparison]
	(operator<=>): Define for C++20.
	* libsupc++/compare (__cmp2way_res_t): Rename to __cmp3way_res_t,
	move into __detail namespace. Do not turn argument types into lvalues.
	(__cmp3way_helper): Rename to __cmp3way_res_impl, move into __detail
	namespace. Constrain with concepts instead of using void_t.
	(compare_three_way_result): Adjust name of base class.
	(compare_three_way_result_t): Use __cmp3way_res_impl directly.
	(__detail::__3way_cmp_with): Add workaround for PR 91073.
	(compare_three_way): Use workaround.
	(__detail::__synth3way, __detail::__synth3way_t): Define new helpers
	implementing synth-three-way and synth-three-way-result semantics.
	* testsuite/20_util/pair/comparison_operators/constexpr_c++20.cc: New
	test.

From-SVN: r278951