Use allows ref struct in comparer interfaces

[stephentoub]

Fixes #103270
Fixes #103323

[ghost]

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[ghost]

Note regarding the new-api-needs-documentation label:

This serves as a reminder for when your PR is modifying a ref *.cs file and adding/modifying public APIs, please make sure the API implementation in the src *.cs file is documented with triple slash comments, so the PR reviewers can sign off that change.

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[stephentoub]

We didn't get to this in API review today. I'm going to go ahead and merge to get it in for P6, and we can back it out if we decide we don't want these (there's no naming discussion here, it's just a "do we or don't we").

[stephentoub]

/ba-g failure is a mono build failure due to file not found