Add the stop count to "statistics dump" in each target's dictionary.

It is great to know how many times the target has stopped over its lifetime as each time the target stops, and possibly resumes without the user seeing it for things like shared library loading and signals that are not notified and auto continued, to help explain why a debug session might be slow. This is now included as "stopCount" inside each target JSON.

Differential Revision: https://reviews.llvm.org/D113810