Auto merge of rust-lang#3742 - Vanille-N:master, r=RalfJung

TB: Reserved + Protected + IM + lazy is a horrible combination that should not exist

As discovered by `@JoJoDeveloping,` the result of having both Protector exceptions on lazy locations (protectors only protect initialized bytes) and interior mutability exceptions for protected tags (Reserved IM does not accept foreign writes when protected) leads to some very undesirable results, namely that we cannot do spurious writes even on protected activated locations.

We propose that Protected Reserved IM should no longer exist and instead when a type is retagged as part of a `FnEntry` it is assumed to lose interior mutability.

In fact, this was already being done implicitly because relevant transitions were guarded by an `if protected`, but the difference is that now it also applies to transitions that occur after the end of the protector.