This gives at least some explanation for why a borrow is expected to
last for a certain free region. Also:
* Reports E0373: "closure may outlive the current function" with NLL.
* Special cases the case of returning a reference to (or value
referencing) a local variable or temporary (E0515).
* Special case assigning a reference to a local variable in a closure
to a captured variable.
It looks like we tend to use angle-brackets around the placeholder in
the few other places we use `Applicability::HasPlaceholders`, but that
would be confusing here, so ...
