Static virtual calls go through task-returning thunks always

[jakobbotsch]

Example from @MichalStrehovsky:

using System;
using System.Runtime.CompilerServices;
using System.Threading.Tasks;
using Xunit;

public class StaticVirtual
{
    interface IHaveStaticVirtuals
    {
        static abstract Task DoTask();
    }

    class ClassWithStaticVirtuals : IHaveStaticVirtuals
    {
        public static async Task DoTask() => await Task.Yield();
    }

    static async Task CallDoTask<T>() where T : IHaveStaticVirtuals => await T.DoTask();

    [Fact]
    public static void TestEntryPoint()
    {
        CallDoTask<ClassWithStaticVirtuals>().Wait();
    }
}

We see task-returning thunk created for DoTask. This is a regression introduced by #124283. We do not properly detect that this is a virtual call.

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[jakobbotsch]

I think we'll want to move the logic that resolves async variants from resolveToken to getCallInfo. resolveToken does not have all the information available it needs to make a good determination about whether the call will be direct or not (specifically for constrained calls).