Made `do_dataflow` and related API `pub(crate)`.

Having the HIR local id is useful for cases like understanding the
ReScope identifiers, which are now derived from the HIR local id, and
thus one can map an ReScope back to the HIR node, once one knows what
those local ids are.

…taflow.

This is meant to ease development of multi-stage dataflow analyses
where the output from one analysis is used to initialize the state
for the next; in such a context, you cannot start with `bottom_value`
for all the bits.

… intrablock state.

(Still musing about whether it could make sense to revise the design
here to make these constraints on usage explicit.)

…ace`.

…d IdxSet.

…hecked.

High-level picture: The old `Borrows` analysis is now called
`Reservations` (implemented as a newtype wrapper around `Borrows`);
this continues to compute whether a `Rvalue::Ref` can reach a
statement without an intervening `EndRegion`. In addition, we also
track what `Place` each such `Rvalue::Ref` was immediately assigned
to in a given borrow (yay for MIR-structural properties!).

The new `ActiveBorrows` analysis then tracks the initial use of any of
those assigned `Places` for a given borrow. I.e. a borrow becomes
"active" immediately after it starts being "used" in some way. (This
is conservative in the sense that we will treat a copy `x = y;` as a
use of `y`; in principle one might further delay activation in such
cases.)

The new `ActiveBorrows` analysis needs to take the `Reservations`
results as an initial input, because the reservation state influences
the gen/kill sets for `ActiveBorrows`. In particular, a use of `a`
activates a borrow `a = &b` if and only if there exists a path (in the
control flow graph) from the borrow to that use. So we need to know if
the borrow reaches a given use to know if it really gets a gen-bit or
not.

 * Incorporating the output from one dataflow analysis into the input
   of another required more changes to the infrastructure than I had
   expected, and even after those changes, the resulting code is still
   a bit subtle.

 * In particular, Since we need to know the intrablock reservation
   state, we need to dynamically update a bitvector for the
   reservations as we are also trying to compute the gen/kills
   bitvector for the active borrows.

 * The way I ended up deciding to do this (after also toying with at
   least two other designs) is to put both the reservation state and
   the active borrow state into a single bitvector. That is why we now
   have separate (but related) `BorrowIndex` and
   `ReserveOrActivateIndex`: each borrow index maps to a pair of
   neighboring reservation and activation indexes.

As noted above, these changes are solely adding the active borrows
dataflow analysis (and updating the existing code to cope with the
switch from `Borrows` to `Reservations`). The code to process the
bitvector in the borrow checker currently just skips over all of the
active borrow bits.

But atop this commit, one *can* observe the analysis results by
looking at the graphviz output, e.g. via

```rust
 #[rustc_mir(borrowck_graphviz_preflow="pre_two_phase.dot",
             borrowck_graphviz_postflow="post_two_phase.dot")]
```

Includes doc for `FindPlaceUses`, as well as `Reservations` and
`ActiveBorrows` structs, which are wrappers are the `Borrows` struct
that dictate which flow analysis should be performed.

two-phase `&mut`-borrow support.

This (new) support sits under `-Z two-phase-borrows` debugflag.

(Still needs tests. That's coming next.)

…blog post on it.

… more surprising.

…evant.

Since we are now checking activation points, I removed one of the
checks at the reservation point. (You can see the effect this had on
two-phase-reservation-sharing-interference-2.rs)

Also, since we now have checks at both the reservation point and the
activation point, we sometimes would observe duplicate errors (since
either one independently interferes with another mutable borrow).  To
deal with this, I used a similar strategy to one used as discussed on
issue rust-lang#45360: keep a set of errors reported (in this case for
reservations), and then avoid doing the checks for the corresponding
activations. (This does mean that some errors could get masked, namely
for conflicting borrows that start after the reservation but still
conflict with the activation, which is unchecked when there was an
error for the reservation. But this seems like a reasonable price to
pay.)

…minators.

(Same net effect as code from before; just cleaner way to get there.)

…of_scope_at_location.

Instead we are "just" careful to invoke it (which sets up a bunch of kill bits)
before we go into the code that sets up the gen bits.

That way, when the gen bits are set up, they will override any
previously set kill-bits for those reservations or activations.

…ror reported at *any* point in past.

…th activations.

…efore activation"

In reality the currently generated MIR has at least one of the activations
in a copy that occurs before the merge. But still, good to have a test,
in anticipation of that potentially changing...

Instead, filter out (non-)conflicts of activiations with themselves in
the same manner that we filter out non-conflict between an activation
and its reservation.