It looks like you can’t access it directly, but you can build it using the framework:
Microsoft.AspNetCore.Http.Extensions.UriHelper.GetFullUrl(Request)
You can also use the above as an extension method.
This returns a string rather than a Uri, but it should serve the purpose! (This also seems to serve the role of the UriBuilder, too.)
Thanks to @mswietlicki for pointing out that it’s just been refactored rather than missing! And also to @C-F to point out the namespace change in my answer!