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.
Give a special message when the later use is from a call. Use the span
of the callee instead of the whole expression. For conflicting borrow
messages say that the later use is of the first borrow.
Instead of tracking the "cause" of each bit that gets added, try to
recover that by walking outlives relationships. This is currently
imprecise, since it ignores the "point" where the outlives relationship
is incurred -- but that's ok, since we're about to stop considering that
overall in a later commit. This does seem to affect one error message
negatively, I didn't dig *too* hard to find out why.
