AK: Resolve clang-tidy warnings about unusual assignment operators

Either not returning *this, or in the case of Variant, not checking for
self assignment. In AK::Atomic, we can't return *this due to the wrapper
semantics Atomic implements.

AK/Checked.h

@@ -138,7 +138,8 @@ public:
     template<typename U>
     constexpr Checked& operator=(U value)
     {
-        return *this = Checked(value);
+        *this = Checked(value);
+        return *this;
     }
 
     constexpr Checked& operator=(const Checked& other) = default;