Fix shared_ptr<T>::owner_before

!(a < b) is not the same as b < a.

I think I originally wrote it this way to avoid making weak_ptr a friend
of shared_ptr, but weak_ptr already is a friend.

ctl/shared_ptr.h

@@ -349,7 +349,7 @@ class shared_ptr
     template<typename U>
     bool owner_before(const weak_ptr<U>& r) const noexcept
     {
-        return !r.owner_before(*this);
+        return rc < r.rc;
     }
 
   private: