Handle calls to shared generic interfaces from shared generic code

[davidwrighton]

• The implementation uses the true LDVIRTFTN path for this. Since it thunks to using function pointer the perf is likely not ideal, but we will need a new generic dictionary entry kind to make this simple, and I'd like to avoid making that change to unblock progress here
• This expensive path was probably fine for use with virtual generic methods, but regular interface dispatch shouldn't be this expensive, so we'll want to fix this later

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[Copilot]

Pull Request Overview

This PR enables calls to shared generic interfaces within shared generic code by routing interface dispatch through the LDVIRTFTN path.

• Implements IGeneric<T> on GenericClass<T> with an explicit Method returning typeof(T)
• Adds a test invocation of the new interface path in TestGenerics_CallsFrom<T>()
• Updates the JIT interpreter (compiler.cpp) to include shared-instantiated classes in the CORINFO_VIRTUALCALL_LDVIRTFTN path

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/tests/JIT/interpreter/Interpreter.cs	Added IGeneric<T> interface; implemented IGeneric<T>.Method in GenericClass<T>; exercised interface dispatch in TestGenerics_CallsFrom<T>()
src/coreclr/interpreter/compiler.cpp	Extended the CORINFO_VIRTUALCALL_LDVIRTFTN case to treat shared-instantiated classes as virtual calls via ldvirtftn

Comments suppressed due to low confidence (2)

src/tests/JIT/interpreter/Interpreter.cs:2143

• The shared-generic interface dispatch path isn’t covered in TestSharedGenerics_CallsTo<T>(). Adding a cast to IGeneric<T> and invoking Method() there would ensure this code path is exercised under shared generics.

    interface IGeneric<T>

src/tests/JIT/interpreter/Interpreter.cs:1899

• [nitpick] The interface method name Method is generic and may be unclear; consider renaming it to something like GetTypeOfT to improve readability.

        Type IGeneric<T>.Method()