Incorrect compiler diagnostic for hidden virtual member

The C# compiler produces an error for this

``` cs
abstract class A
{
public abstract void M();
}
abstract class B: A
{
// (?) It is an error to hide an inherited abstract member by a
// member whose access level is public, protected internal, or protected.
public new int M = 0; // error: B.M hides inherited member A.M()
}
```

But the error is not justified by anything (that I could find) in the C# language specification. We should either describe the required error in the C# specification or modify the compiler to stop producing it.

@AlekseyTs @MadsTorgersen