LibJS: Store PrivateElement values in Handle<Value>

This fixes an issue where private element values were not always
protected from GC. I found two instances where this was happening:

- ECMAScriptFunctionObject did not mark m_private_methods
- ClassDefinitionEvaluation had two Vector<PrivateElement> that were
  opaque to the garbage collector, and so if GC occurred while
  constructing a class instance, some or all of its private elements
  could get incorrectly collected.

Userland/Libraries/LibJS/AST.cpp

@@ -1735,13 +1735,13 @@ ThrowCompletionOr<ClassElement::ClassValue> ClassMethod::class_element_evaluatio
         switch (kind()) {
         case Kind::Method:
             set_function_name();
-            return ClassValue { PrivateElement { private_name, PrivateElement::Kind::Method, method_value } };
+            return ClassValue { PrivateElement { private_name, PrivateElement::Kind::Method, make_handle(method_value) } };
         case Kind::Getter:
             set_function_name("get");
-            return ClassValue { PrivateElement { private_name, PrivateElement::Kind::Accessor, Accessor::create(interpreter.vm(), &method_function, nullptr) } };
+            return ClassValue { PrivateElement { private_name, PrivateElement::Kind::Accessor, make_handle(Value(Accessor::create(interpreter.vm(), &method_function, nullptr))) } };
         case Kind::Setter:
             set_function_name("set");
-            return ClassValue { PrivateElement { private_name, PrivateElement::Kind::Accessor, Accessor::create(interpreter.vm(), nullptr, &method_function) } };
+            return ClassValue { PrivateElement { private_name, PrivateElement::Kind::Accessor, make_handle(Value(Accessor::create(interpreter.vm(), nullptr, &method_function))) } };
         default:
             VERIFY_NOT_REACHED();
         }
@@ -2043,11 +2043,11 @@ ThrowCompletionOr<ECMAScriptFunctionObject*> ClassExpression::class_definition_e
                 if (existing.key == private_element.key) {
                     VERIFY(existing.kind == PrivateElement::Kind::Accessor);
                     VERIFY(private_element.kind == PrivateElement::Kind::Accessor);
-                    auto& accessor = private_element.value.as_accessor();
+                    auto& accessor = private_element.value.value().as_accessor();
                     if (!accessor.getter())
-                        existing.value.as_accessor().set_setter(accessor.setter());
+                        existing.value.value().as_accessor().set_setter(accessor.setter());
                     else
-                        existing.value.as_accessor().set_getter(accessor.getter());
+                        existing.value.value().as_accessor().set_getter(accessor.getter());
                     added_to_existing = true;
                 }
             }

Userland/Libraries/LibJS/Runtime/Object.cpp

@@ -508,7 +508,7 @@ ThrowCompletionOr<void> Object::private_field_add(PrivateName const& name, Value
         m_private_elements = make<Vector<PrivateElement>>();
 
     // 4. Append PrivateElement { [[Key]]: P, [[Kind]]: field, [[Value]]: value } to O.[[PrivateElements]].
-    m_private_elements->empend(name, PrivateElement::Kind::Field, value);
+    m_private_elements->empend(name, PrivateElement::Kind::Field, make_handle(value));
 
     // 5. Return unused.
     return {};
@@ -559,14 +559,14 @@ ThrowCompletionOr<Value> Object::private_get(PrivateName const& name)
     // 3. If entry.[[Kind]] is either field or method, then
     if (entry->kind != PrivateElement::Kind::Accessor) {
         // a. Return entry.[[Value]].
-        return value;
+        return value.value();
     }
 
     // Assert: entry.[[Kind]] is accessor.
-    VERIFY(value.is_accessor());
+    VERIFY(value.value().is_accessor());
 
     // 6. Let getter be entry.[[Get]].
-    auto* getter = value.as_accessor().getter();
+    auto* getter = value.value().as_accessor().getter();
 
     // 5. If entry.[[Get]] is undefined, throw a TypeError exception.
     if (!getter)
@@ -591,7 +591,7 @@ ThrowCompletionOr<void> Object::private_set(PrivateName const& name, Value value
     // 3. If entry.[[Kind]] is field, then
     if (entry->kind == PrivateElement::Kind::Field) {
         // a. Set entry.[[Value]] to value.
-        entry->value = value;
+        entry->value = make_handle(value);
         return {};
     }
     // 4. Else if entry.[[Kind]] is method, then
@@ -606,10 +606,10 @@ ThrowCompletionOr<void> Object::private_set(PrivateName const& name, Value value
     VERIFY(entry->kind == PrivateElement::Kind::Accessor);
 
     auto& accessor = entry->value;
-    VERIFY(accessor.is_accessor());
+    VERIFY(accessor.value().is_accessor());
 
     // c. Let setter be entry.[[Set]].
-    auto* setter = accessor.as_accessor().setter();
+    auto* setter = accessor.value().as_accessor().setter();
 
     // b. If entry.[[Set]] is undefined, throw a TypeError exception.
     if (!setter)
@@ -1350,11 +1350,6 @@ void Object::visit_edges(Cell::Visitor& visitor)
     m_indexed_properties.for_each_value([&visitor](auto& value) {
         visitor.visit(value);
     });
-
-    if (m_private_elements) {
-        for (auto& private_element : *m_private_elements)
-            visitor.visit(private_element.value);
-    }
 }
 
 // 7.1.1.1 OrdinaryToPrimitive ( O, hint ), https://tc39.es/ecma262/#sec-ordinarytoprimitive

Userland/Libraries/LibJS/Runtime/Object.h

@@ -36,7 +36,7 @@ struct PrivateElement {
 
     PrivateName key;
     Kind kind { Kind::Field };
-    Value value;
+    Handle<Value> value;
 };
 
 class Object : public Cell {