`trait Trait: Send`: upcasting from `dyn Trait` to `dyn Send` does not work

[lcnr]

The following compiles:

#![feature(trait_upcasting)]
trait Super {}
trait Trait: Super {}
fn foo(x: &dyn Trait) -> &dyn Super { x }

while this does not:

#![feature(trait_upcasting)]
trait Super {}
trait Trait: Send {}
fn foo(x: &dyn Trait) -> &dyn Send { x }

cc @compiler-errors I thought we actually had a PR for that at some point 🤔 I think that's actually not the case though. It wasn't part of #119338

[compiler-errors]

I had a separate PR to upcast dropping the principal but I think I never FCPd it because I was too lazy: #114679 (comment)