Stacked Borrows: Show stacktrace of when an item was popped

[RalfJung]

When debugging a stacked borrows failure, it would be really useful if, together with the message saying an item was not found in the stack, one could get a stacktrace saying "here is when the item got popped". That would indicate the conflicting access that invalidated the access that is right now being performed.

[RalfJung]

With -Zmiri-track-pointer-tag we have a crude approximation of this now (that only works for deterministic executions).

[saethlin]

Would you consider this closed by #2030 ?

[RalfJung]

Yes. :-)