Auto merge of #52168 - nikomatsakis:nll-region-name, r=estebank

find and highlight the `&` or `'_` in `region_name`

Before:

```
   --> $DIR/dyn-trait-underscore.rs:18:5
    |
 LL | fn a<T>(items: &[T]) -> Box<dyn Iterator<Item=&T>> {
-   |         ----- lifetime `'1` appears in this argument
 LL |     Box::new(items.iter()) //~ ERROR cannot infer an appropriate lifetime
    |     ^^^^^^^^^^^^^^^^^^^^^^ cast requires that `'1` must outlive `'static`
```

After:

```
   --> $DIR/dyn-trait-underscore.rs:18:5
    |
 LL | fn a<T>(items: &[T]) -> Box<dyn Iterator<Item=&T>> {
+   |                - let's call the lifetime of this reference `'1`
 LL |     Box::new(items.iter()) //~ ERROR cannot infer an appropriate lifetime
    |     ^^^^^^^^^^^^^^^^^^^^^^ cast requires that `'1` must outlive `'static`
```

Not intended as the final end point necessarily in any sense. I intentionally left some to-do points to fill in later:

- Does not apply to upvars in closures yet (should be relatively easy)
- Does not handle the case where we can't find a precise match very well
- And of course we can still tweak wording

but shows the basic idea of how to make the `Ty` and `hir::Ty` to find a good spot to highlight.

r? @estebank
cc @davidtwco

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