There is no longer need for MvcHtmlString (which added on top of HtmlString for compatibility reasons) so you can simply use Microsoft.AspNet.Mvc.Rendering.HtmlString.
As you can easily notice, the ViewComponents and Helpers of Asp.Net 5 are also using (by returning) the same HtmlString to the views.
Update (for ASP.NET Core 3.1):
You need to use Microsoft.AspNetCore.Html.HtmlString instead.