Warn if ContractContractValidator doesn't have a validation logic

[SergeyTeplyakov]

Contract validation method marked with ContractArgumentValidator should follow few rules:

1. Guard method should has a if-throw
2. if-throw should follow Contract.EndContractBlock

So following code will emit the warning:

using System.Diagnostics.Contracts;
public static class ContractValidator
{
    [ContractArgumentValidator]
    public static void Guard(bool condition)
    {
      // warning CC1053: No validation code recognized in contract argument 
      // validator 'ContractValidator.Guard(System.Boolean)'.
      // if (!condition)
        throw new Exception();
      Contract.EndContractBlock();
    }
}

The same warning would be emitted if uncomment if-condition and comment out Contract.EndContractBlock().