AK: Mark smart pointer classes as [[nodiscard]]

This makes it an error to not do something with a returned smart
pointer, which should help prevent mistakes. In cases where you do need
to ignore the value, casting to void will placate the compiler.

I did have to add comments to disable clang-format on a couple of lines,
where it wanted to format the code like this:

```c++
private : NonnullRefPtr() = delete;
```

AK/RefPtr.h

@@ -27,7 +27,7 @@ template<typename T>
 class OwnPtr;
 
 template<typename T, typename PtrTraits>
-class RefPtr {
+class [[nodiscard]] RefPtr {
     template<typename U, typename P>
     friend class RefPtr;
     template<typename U>