[NLL] Loss of suggestions to change/fix trait object lifetime bound

[marmeladema]

For tests like:

• src/test/ui/regions/regions-close-object-into-object-2.rs

In NLL mode, we loose suggestions to change trait object lifetime bound like:

help: consider changing the trait object's explicit `'static` bound to the lifetime of argument `v`
   |
LL | fn g<'a, T: 'static>(v: Box<dyn A<T> + 'a>) -> Box<dyn X + 'a> {
   |                                                            ~~

or

help: alternatively, add an explicit `'static` bound to this reference
   |
LL | fn g<'a, T: 'static>(v: Box<(dyn A<T> + 'static)>) -> Box<dyn X + 'static> {
   |                         ~~~~~~~~~~~~~~~~~~~~~~~~~

The latter mentions a reference which is a bit wrong in this case since only Box-es are involved but it can probably be solved separately.
In any case, we should re-instate those suggestions in NLL mode.

@rustbot label +A-NLL +NLL-diagnostics