Remove class constraint on AsNonNull()

[smoogipoo]

This change is the only exact combination of things I can find that replicates the inspections provided by !.

My tests are:

#nullable enable
    object a = new object();
    object a1 = a!;
    object a2 = a.AsNonNull();
    Debug.Assert(a1 == null);
    Debug.Assert(a2 == null);

    object? b = new object();
    object b1 = b!;
    object b2 = b.AsNonNull();
    Debug.Assert(b1 == null);
    Debug.Assert(b2 == null);

    object? c = null;
    object c1 = c!;
    object c2 = c.AsNonNull();
    Debug.Assert(c1 == null);
    Debug.Assert(c2 == null);

    int d = 5;
    int d1 = d!;
    int d2 = d.AsNonNull();
    Debug.Assert(d1 == null);
    Debug.Assert(d2 == null);

    int? e = 5;
    int? e1 = e!;
    int? e2 = e.AsNonNull();
    Debug.Assert(e1 == null);
    Debug.Assert(e2 == null);

    int? f = null;
    int? f1 = f!;
    int? f2 = f.AsNonNull();
    Debug.Assert(f1 == null);
    Debug.Assert(f2 == null);
#nullable disable

Here's a comparison leading to my results. Open each image in a tab to see the differences.

Master: https://user-images.githubusercontent.com/1329837/177527783-e7616647-4a80-488c-8a04-83aa02c60919.png
Removing the T type: https://user-images.githubusercontent.com/1329837/177527683-a5b72f6e-21b3-4420-99c0-287c9bc5f456.png
Trying [return: NotNull]: https://user-images.githubusercontent.com/1329837/177528220-c924e752-ec75-43b5-80ab-caf94f4c775e.png
Using [return: NotNullIfNotNull("obj")]: https://user-images.githubusercontent.com/1329837/177528133-07c0629d-eba0-4e92-ba70-504c37e60948.png

[frenzibyte]

Unfortunate that the attribute requires literal string since nameof isn't supported on parameter names, there seem to be proposals about it as well (dotnet/csharplang#373 (comment)).