### 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)


update_const_expression_builtin_type(p_return->return_value, expected_type, "return");
			}
			result = p_return->return_value->get_datatype();
		}
	} else {
		
		result.type_source = GDScriptParser::DataType::ANNOTATED_EXPLICIT;
		result.kind = GDScriptParser::DataType::BUILTIN;
		result.builtin_type = Variant::NIL;
		result.is_constant = true;
	}
	if (has_expected_type && !expected_type.is_variant()) {
		if (result.is_variant() || !result.is_hard_type()) {
			mark_node_unsafe(p_return);
			if (!is_type_compatible(expected_type, result, true, p_return)) {
				downgrade_node_type_source(p_return);
			}
		} else if (!is_type_compatible(expected_type, result, true, p_return)) {
			mark_node_unsafe(p_return);
			if (!is_type_compatible(result, expected_type)) {
				push_error(vformat(R"(Cannot return value of type "%s" because the function return type is "%s".)", result.to_string(), expected_type.to_string()), p_return);
			}
#ifdef DEBUG_ENABLED
		} else if (expected_type.builtin_type == Variant::INT && result.builtin_type == Variant::FLOAT) {
			parser->push_warning(p_return, GDScriptWarning::NARROWING_CONVERSION);
#endif
		}
	}
	p_return->set_datatype(result);
}