[Swift language feature] Implement generic return type in Swift async functions

[kotlarmilos]

Description

We need to implement AOT compatible C# async callback. Here is an example:

[UnmanagedCallersOnly(CallConvs = new[] { typeof(CallConvCdecl) })]
private static void GenericCollectionConstraintOnComplete(System.IntPtr rawResult, IntPtr task)
{
    GCHandle handle = GCHandle.FromIntPtr(task);
    try
    {

        var result = SwiftMarshal.MarshalFromSwift<System.IntPtr>((SwiftHandle)new IntPtr(&rawResult));
        if (handle.Target is TaskCompletionSource<System.IntPtr> tcs)
        {
            tcs.TrySetResult(result);
        }
    }
    finally
    {
        handle.Free();
    }
}

The rawResult parameter should be generic type, which is not allowed in UnmanagedCallersOnly methods.