[lldb] Refine ThreadPlan::ShouldAutoContinue

Adjust `ShouldAutoContinue` to be available to any thread plan previous to the plan that
explains a stop, not limited to the parent to the plan that explains the stop.

Before this change, `Thread::ShouldStop` did the following:

1. find the plan that explains the stop
2. if it's not a master plan, continue processing previous (aka parent) plans
3. first, call `ShouldAutoContinue` on the immediate parent of the explaining plan
4. then loop over previous plans, calling `ShouldStop` and `MischiefManaged`

Of note, the iteration in step 4 does not call `ShouldAutoContinue`, so again only the
plan just prior to the explaining plan is given the opportunity to override whether to
continue or stop.

This commit changes the loop call `ShouldAutoContinue`, giving each plan the opportunity
to override `ShouldStop` of previous plans.

Why? This allows a plan to do the following:

1. mark itself done and be popped off the stack
2. allow parent plans to finish their work, a...