Fine tune dianostics for when a borrow conflicts with a destructor that needs exclusive access.

In particular:

 1. Extend `WriteKind::StorageDeadOrDrop` with state to track whether
    we are running a destructor or just freeing backing storage.  (As
    part of this, when we drop a Box<..<Box<T>..> where `T` does not
    need drop, we now signal that the drop of `T` is a kind of storage
    dead rather than a drop.)

 2. When reporting that a value does not live long enough, check if
    we're doing an "interesting" drop, i.e. we aren't just trivally
    freeing the borrowed state, but rather a user-defined dtor will
    run and potentially require exclusive aces to the borrowed state.

 3. Added a new diagnosic to describe the scenario here.

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