libstdc++: Add comparison operators to std::unique_ptr

libstdc++: Add comparison operators to std::unique_ptr

Some more C++20 changes from P1614R2, "The Mothership has Landed".

This includes the proposed resolution for LWG 3426 to fix the three-way
comparison with nullptr_t.

The existing tests for unique_ptr comparisons don't actually check the
results, only that the expressions compile and are convertible to bool.
This also adds a test for the results of those comparisons for C++11 and
up.

	* include/bits/unique_ptr.h (operator<=>): Define for C++20.
	* testsuite/20_util/default_delete/48631_neg.cc: Adjust dg-error line.
	* testsuite/20_util/default_delete/void_neg.cc: Likewise.
	* testsuite/20_util/unique_ptr/comparison/compare.cc: New test.
	* testsuite/20_util/unique_ptr/comparison/compare_c++20.cc: New test.