Add support for generic attributes

[sbomer]

This adds support for generic attributes, added in C# 10. This is needed to fix a linker crash on runtime tests that used generic attributes: dotnet/linker#2963

Upstreaming this in jbevain#871.

[sbomer]

@MichalStrehovsky @vitek-karas @marek-safar PTAL (I can't add reviewers)

[MichalStrehovsky]

Not an expert, but looks reasonable. Nice that it already worked for the named arguments case.

Does the test also test writing? Though, I'm not too concerned about that since I assume we'll have writing coverage in the linker repo.

[sbomer]

Yes, TestModule will test writing. I didn't look too closely at what exactly it validates, but it does some kind of round-tripping, and was hitting the same crash in the writing code before the fix.

[sbomer]

@marek-safar would you be able to merge this? I don't have permission. Thanks!