Fix raciness in the StopHook check for "has the target run".

This was looking at the privateState, but it's possible that
the actual process has started up and then stopped again by the
time we get to the check, which would lead us to get out of running
the stop hooks too early.

Instead we need to track the intention of the stop hooks directly.

Differential Revision: https://reviews.llvm.org/D88753