func isStructUnionClass(x ast.Expr) bool {
	id, ok := x.(*ast.Ident)
	if !ok {
		return false
	}
	name := id.Name
	return strings.HasPrefix(name, "_Ctype_struct_") ||
		strings.HasPrefix(name, "_Ctype_union_") ||
		strings.HasPrefix(name, "_Ctype_class_")
}

// FuncArg returns a Go type with the same memory layout as
// dtype when used as the type of a C function argument.