libstdc++: Make all experimental::observer_ptr functions constexpr

libstdc++: Make all experimental::observer_ptr functions constexpr

I've just created LWG 4295 proposing this change, and am implementing it
via this patch.

libstdc++-v3/ChangeLog:

	* include/experimental/memory (swap, make_observer_ptr): Add
	constexpr.
	(operator==, operator!=, operator<, operator>, operator<=)
	(operator>=): Likewise.
	* testsuite/experimental/memory/observer_ptr/make_observer.cc:
	Checks for constant evaluation.
	* testsuite/experimental/memory/observer_ptr/relops/relops.cc:
	Likewise.
	* testsuite/experimental/memory/observer_ptr/swap/swap.cc:
	Likewise.

Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>