[Default Interfaces] Support constrained interface calls on value types

[MichalStrehovsky]

Pull request dotnet/coreclr#21978 made us throw in this situation, but the correct behavior would be to make a thunk that will box the this, and dispatch to the target default interface method.

[gafter]

The following code compiles and runs without throwing. Not sure what scenario is not handled properly.

using System;

class Program
{
    static void Main(string[] args)
    {
        S1 s = new S1();
        M(s);
    }

    static void M<T>(T t) where T : I1
    {
        t.M();
    }
}

interface I1
{
    void M() { Console.WriteLine("I1.M"); }
}

struct S1 : I1
{
}

[MichalStrehovsky]

It's specifically for the corner case mentioned in the pull request (there is an il test too).

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