LibJS: Clarify Object (base class) construction somewhat

Divide the Object constructor into three variants:

- The regular one (takes an Object& prototype)
- One for use by GlobalObject
- One for use by objects without a prototype (e.g ObjectPrototype)

Libraries/LibJS/Runtime/Object.h

@@ -62,7 +62,7 @@ class Object : public Cell {
 public:
     static Object* create_empty(Interpreter&, GlobalObject&);
 
-    explicit Object(Object* prototype);
+    explicit Object(Object& prototype);
     virtual void initialize(Interpreter&, GlobalObject&) override;
     virtual ~Object();
 
@@ -142,6 +142,12 @@ public:
 
     Value invoke(const FlyString& property_name, Optional<MarkedValueList> arguments = {});
 
+protected:
+    enum class GlobalObjectTag { Tag };
+    enum class ConstructWithoutPrototypeTag { Tag };
+    explicit Object(GlobalObjectTag);
+    Object(ConstructWithoutPrototypeTag, GlobalObject&);
+
 private:
     virtual Value get_by_index(u32 property_index) const;
     virtual bool put_by_index(u32 property_index, Value);