[NLL] Loss of span for `consider adding an explicit lifetime bound` suggestions

[marmeladema]

In tests like:

• src/test/ui/regions/regions-close-object-into-object-5.rs
• src/test/ui/regions/regions-close-over-type-parameter-multiple.rs

Under NLL we rightfully suggest to add a lifetime bound:

   |
LL |     Box::new(B(&*v)) as Box<dyn X>
   |              ^^^^^^
   |
   = help: consider adding an explicit lifetime bound `T: 'static`...

but the actual span where the lifetime bound should be added is lost when we compare to non-nll mode:

LL | fn f<'a, T, U>(v: Box<A<T> + 'static>) -> Box<X + 'static> {
   |          - help: consider adding an explicit lifetime bound...: `T: 'static`

This is a diagnostic regression that should be fixed.

@rustbot label +A-NLL +NLL-diagnostics

[jackh726]

I think this can be closed now, after #96352