[DllImportGenerator] warn on non-partial class's parents

[SupinePandora43]

public class Something {
    internal static partial class PInvokes {
        [GeneratedDllImport("Example")]
        internal static partial void DoSomething([MarshalAs(UnmanagedType.I1)] bool enable);
    }
}

Expected result

warning

Actual result

CS0101: The namespace 'MyNamespace' already contains a definition for 'Something'

[jkoritzinsky]

Fixed by #1091