Added `-Z two-phase-beyond-autoref` to bring back old behavior (mainly
to allow demonstration of desugared examples).
Updated tests to use aforementioned flag when necessary. (But in each
case where I added the flag, I made sure to also include a revision
without the flag so that one can readily see what the actual behavior
we expect is for the initial deployment of NLL.)
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 #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.)
