### Godot version

Beta 17, master

### System information

Windows 11

### Issue description

Godot gets a hard crash while attempting to code-complete for member `const` of a class.

### Steps to reproduce

```GDScript
enum E { V1 }

const e := E
```

And then type a `.` after the last `E`, which triggers code hints/auto-completion for subscripts based on `E` and causes a hard crash.

Does not happen for `var e := E.V1`.

### Minimal reproduction project

See above.
reduce_expression(p_cast->operand);
	GDScriptParser::DataType cast_type = type_from_metatype(resolve_datatype(p_cast->cast_type));
	if (!cast_type.is_set()) {
		mark_node_unsafe(p_cast);
		return;
	}
	p_cast->set_datatype(cast_type);
	if (p_cast->operand->is_constant) {
		update_const_expression_builtin_type(p_cast->operand, cast_type, "cast", true);
		if (cast_type.is_variant() || p_cast->operand->get_datatype() == cast_type) {
			p_cast->is_constant = true;
			p_cast->reduced_value = p_cast->operand->reduced_value;
		}