Warn for using less accessible members in guards

[SergeyTeplyakov]

Warn for using less accessible members in guard methods marked with `ContractArgumentValidator':

public static class ContractValidator
{
    [ContractArgumentValidator]
    public static void Guard(bool condition)
    {
        if (condition)
            throw new Exception();
        Contract.EndContractBlock();
    }
}

class Sample
{
  public void Foo(string s)
  {
    ContractValidator.Guard(s != null);
  }
}

Note, that Code Contract compiler doesn't warn about this kind of issue and warn only when contract validation method itself uses less accessible members.