Consider detecting unnecessary annotations

[vitek-karas]

This came as feedback from the team annotating libraries. It's likely that libraries contain unnecessary annotations, specifically RequiredUnreferencedCode and possibly even unnecessary suppressions UnconditionalSuppressMessage.
This can happen for multiple reasons, the most common ones:

• Incorrect understanding of warnings and behavior of the linker, leading to over-annotation the code.
• Improvements in the linker - code which needed annotation/suppression is fully recognized now
• Simple bugs
• Changes to the code which make existing annotations/suppressions stale

Ideally this would be implemented in the Roslyn analyzer as code fixes/suggestions - definitely not warnings.
But until then, linker can probably figure out good chunk of these as well and could report them as simple info-messages to the output.

[marek-safar]

I think it could be a new linker option which we someone would enable from time to time to clean them up in libraries

[vitek-karas]

Potentially linker could detect unnecessary DynamicallyAccessedMembers annotations as well.

[jtschuster]

dotnet/linker#2922 Added IL2121 to the linker to warn on redundant suppressions. Looks like it's not supported in ilc or the trim analyzer.

[vitek-karas]

The ilc part is already tracked in #75201.

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