### Godot version

4.1.beta3

### System information

Fedora Workstation 38 - Godot 4.1.beta3 - Vulkan (Forward+)

### Issue description

**This is an issue created by the production team, we're working on it**

Currently, it's impossible to call virtual functions within GDScript.

This currently returns an error:
```gdscript
# a.gd
extends Node

func _process(delta: float) -> void:
	pass
```

```gdscript
# main.gd
extends Node

var my_node: Node = preload("res://a.gd").new()

func _process(delta: float) -> void:
	my_node._process(delta)  # Cannot call virtual function "_process()" because it hasn't been defined.
```

### Steps to reproduce

1. Create a script (`a.gd`) based on `Node` that declares `_process()`;
2. Create a main node;
3. Attach a script to the main node;
4. Initialize a variable with a new `a.gd` instance;
5. Try to run `_process()` on that node;
6. Find out that it returns an error.

### Minimal reproduction project

[virtual_functions.zip (updated)](https://github.com/godotengine/godot/files/11823992/virtual_functions.zip)


GDScriptParser::IdentifierNode *identifier = nullptr;
	if (p_node->type == GDScriptParser::Node::IDENTIFIER) {
		identifier = static_cast<GDScriptParser::IdentifierNode *>(p_node);
	} else if (p_node->type == GDScriptParser::Node::SUBSCRIPT) {
		GDScriptParser::SubscriptNode *subscript = static_cast<GDScriptParser::SubscriptNode *>(p_node);
		if (subscript->is_attribute) {
			identifier = subscript->attribute;
		}
	}
	if (identifier == nullptr) {
		return;
	}
	GDScriptParser::Node *source = nullptr;
	switch (identifier->source) {
		case GDScriptParser::IdentifierNode::MEMBER_VARIABLE: {
			source = identifier->variable_source;
		} break;
		case GDScriptParser::IdentifierNode::FUNCTION_PARAMETER: {
			source = identifier->parameter_source;
		} break;
		case GDScriptParser::IdentifierNode::LOCAL_VARIABLE: {
			source = identifier->variable_source;
		} break;
		case GDScriptParser::IdentifierNode::LOCAL_ITERATOR: {
			source = identifier->bind_source;
		} break;
		default:
			break;