Attribute stripping

[MichalStrehovsky]

With #29723 looking more and more real, I think we need to think about how attribute stripping can be done safely.

Attribute stripping is currently the only optimization in illink that I know of that will break user code without any warnings.

I think it would be interesting if we could scope attribute stripping down to places where we can provably do it safely. Illink has a good idea of what members are going to be reflected on. We could limit attribute stripping to places that we know are not going to be reflected on. I think we could still get pretty decent savings and APIs like the proposed nullability info decoder would not get broken by illink.

Cc @vitek-karas

[marek-safar]

Duplicate of dotnet/linker#1991

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