We never have more than one imaginary target, so we have no reason for
a `Vec`

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`

This also fully stabilizes two-phase borrows on all editions

A comment in one match arm make a blanket statement about "reads/reservations", but in fact the whole point of this PR is that reservations are *not* handled by that particular arm anymore.

The `InlineAsm` variant is extremely rare, and `mir::Statement` often
contributes significantly to peak memory usage.

Enum layout optimizations mean that the discriminant of an enum may not
be stored in a tag disjoint from the rest of the fields of the enum.
Stop borrow checking as though they are.

Since lexical MIR borrow check is gone, and validation no longer uses
these, they can be removed.

Also "rename" -Zmir-emit-validate to -Zmir-emit-retag, which is just a boolean (yes or no).