AK: Really disallow making OwnPtrs from refcounted types

This looks at three things:
- if the type has a typedef `AllowOwnPtr', respect that
- if not, disallow construction if both of `ref()' and `unref()' are
  present.
Note that in the second case, if a type only defines `ref()' or only
defines `unref()', an OwnPtr can be created, as a RefPtr of that type
would be ill-formed.

Also marks a `Performance' to explicitly allow OwnPtrs.

AK/OwnPtr.h

@@ -38,7 +38,9 @@ public:
     explicit OwnPtr(T* ptr)
         : m_ptr(ptr)
     {
-        static_assert(!is_ref_counted((const T*)nullptr), "Use RefPtr<> for RefCounted types");
+        static_assert(
+            requires { requires typename T::AllowOwnPtr()(); } || !requires(T obj) { requires !typename T::AllowOwnPtr()(); obj.ref(); obj.unref(); },
+            "Use RefPtr<> for RefCounted types");
     }
     OwnPtr(OwnPtr&& other)
         : m_ptr(other.leak_ptr())