The liveness dataflow analysis now lives in
`librustc_mir/dataflow/impls/liveness.rs`. The borrow-checker has an
abstraction around of "defs" and "uses" that I've made module private. I
would have moved it to `util/def_use.rs`, but there's a slightly
different abstraction used for copy propagation with that name.

The commit should have changed comments as well.
At the time of writting, it passes the tidy and check tool.

Revisions asked by eddyb :
- Renamed of all the occurences of {visit/super}_mir
- Renamed test structures `CachedMir` to `Cached`

Fixing the missing import on `AggregateKind`

Previously, explain_borrow would emit an error with the explanation of
the a borrow. Now, it returns a enum with what the explanation for the
borrow is and any relevant spans or information such that the calling
code can choose to emit the same note/suggestion as before by calling
the emit method on the new enum.

Less modes!

The strategy is this:

- we compute SCCs once all outlives constraints are known
- we allocate a set of values **per region** for storing liveness
- we allocate a set of values **per SCC** for storing the final values
- when we add a liveness constraint to the region R, we also add it
  to the final value of the SCC to which R belongs
- then we can apply the constraints by just walking the DAG for the
  SCCs and union'ing the children (which have their liveness
  constraints within)

There are a few intermediate refactorings that I really ought to have
broken out into their own commits:

- reverse the constraint graph so that `R1: R2` means `R1 -> R2` and
  not `R2 -> R1`. This fits better with the SCC computation and new
  style of inference (`->` now means "take value from" and not "push
  value into")
  - this does affect some of the UI tests, since they traverse the
    graph, but mostly the artificial ones and they don't necessarily
    seem worse
- put some things (constraint set, etc) into `Rc`. This lets us root
  them to permit mutation and iteration. It also guarantees they don't
  change, which is critical to the correctness of the algorithm.
- Generalize various helpers that previously operated only on points
  to work on any sort of region element.