Stop rooting nested types for helper types

[Sergio0694]

While looking at sizoscope traces with @jkoritzinsky, he noticed that the linker was rooting a considerable amount of code due to virtual methods on nested types for helper types, ie. all of the vtable types. These vtable types are no longer used at all with new projections (and NAOT will not even attempt to find them at all, and will just fail if using old projections). This PR updates the trimming annotations to stop rooting all of these nested vtable types, making the fallback path using older projections not trim-safe. It will still work though, and we added some explicit annotations to preserve the generic vtable types that are generally needed by applications, only on CoreCLR, so we should be able to maintain back-compat there when trimming is used. This makes the back-compat support pay-for-play and avoid the binary size increase for customers using new projections and NAOT especially.

See:

[dongle-the-gadget]

This would break over existing trimmed users.

[Sergio0694]

Yes, but we could detail that in our release notes, and it would only break users of other 3rd party WinRT components that don't get updated in sync (which combined with the users of C#, and using trimming, makes for a very small number of users). Those users can wait to make sure their dependencies are updated before bumping CsWinRT, or alternatively they can make sure to manually root those projection assemblies as a temporary workaround. But this way we can make this pay for play.

[Sergio0694]

Saves 213 KB 🎉

[manodasanW]

Confirmed types don't get trimmed when building with old projections.