Use allows ref struct in comparer interfaces

[stephentoub]

Fixes #103270
Fixes #103323

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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