Auto merge of rust-lang#3754 - Vanille-N:master, r=RalfJung

Make unused states of Reserved unrepresentable

In the [previous TB update](rust-lang/miri#3742) we discovered that the existence of `Reserved + !ty_is_freeze + protected` is undesirable.

This has the side effect of making `Reserved { conflicted: true, ty_is_freeze: false }` unreachable.
As such it is desirable that this state would also be unrepresentable.

This PR eliminates the unused configuration by changing
```rs
enum PermissionPriv {
    Reserved { ty_is_freeze: bool, conflicted: bool },
    ...
}
```
into
```rs
enum PermissionPriv {
    ReservedFrz { conflicted: bool },
    ReservedIM,
    ...
}
```
but this is not the only solution and `Reserved(Activable | Conflicted | InteriorMut)` could be discussed.
In addition to making the unreachable state not representable anymore, this change has the nice side effect of enabling `foreign_read` to no longer depend explicitly on the `protected` flag.

Currently waiting for
- `@JoJoDeveloping` to confirm that this is the same representation of `Reserved` as what is being implemented in simuliris,
- `@RalfJung` to approve that this does not introduce too much overhead in the trusted codebase.