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 is worth pointing out that the reason that so few diagnostics are
effected is because of the filter I put in where it only goes down the
new path if the borrowed place is *not* a prefix of the dropped place.
(Without that filter, a *lot* of the tests would need this change, and
it would probably be a net loss for the UX, since you'd see it even in
cases like borrows of generic types where there is no explicit mention
of `Drop`.)
