Ability to step backwards until a value is assigned

[jimgrundy]

One begins counter-example debugging from a failing state. Often when I look at one of these I see a variable with a value that I feel it shouldn't have. "How did that happen?" is the question I ask myself at this point. A handy debug feature then to be able to roll back to the earlier point at which the variable took on that value. If there were a way I could select a value and automatically go to the place and time where that value came into existence I think it would help. An analogous feature is part of hardware model checking counter-example visualization tools.

[tautschnig]

We had previously discussed this in the context of viewer, and diffblue/cbmc#4914 came up to perhaps have sufficient information in the trace. @markrtuttle will we still need such information to make this feasible in the VS code implementation?