Fix lifetime error to print `'a` instead of `&'a`

This changes certain error messages about lifetimes so that they display
lifetimes without an `&`.

Fixes #10291.
auto merge of #14317 : P1start/rust/lifetime-formatting, r=alexcrichton

This changes certain error messages about lifetimes so that they display lifetimes without an `&`.

Fixes #10291.