Function resolve_class_of_arrow_access

* For every lhs before the arrow ('$a->...', '(new A())->...', 'get()->nestedArr[0]->...'), * we expect some class instance on the left side. * Deduce that class and throw a meaningful error if something went wrong. * For example, the code '$a = 42; $a->...' will result in '$a is not an instance' error. */