Missing warnings for dynamically accessed members in presence of interfaces

```csharp
using System;
using System.Diagnostics.CodeAnalysis;
using DAM = System.Diagnostics.CodeAnalysis.DynamicallyAccessedMembersAttribute;
using RUC = System.Diagnostics.CodeAnalysis.RequiresUnreferencedCodeAttribute;

public class Program {
public static void Main() => CallMethod(typeof(Foo), new Foo());

static void CallMethod([DAM(DynamicallyAccessedMemberTypes.PublicMethods)] Type t, object i) {
t.GetMethod("Method").Invoke(i, null);
}
}

public interface I {
[RUC("RUC")]
void Method();
}

public class Foo : I {
[RUC("RUC")]
public void Method() {
throw new Exception();
}
}
```

This doesn't produce any warnings for the dynamic call to the RUC method. Note that it does warn if you remove Foo's implementation of I. I think this check should be modified to warn on reflection access to newslot virtuals: https://github.com/mono/linker/blob/6eae01980dc694107bdee0bc723d75a0dd601f0e/src/linker/Linker.Steps/MarkStep.cs#L2787-L2792