[Windows][Backtracing] Don't use `DebugActiveProcess()`.
`DebugActiveProcess()` can fail under certain circumstances, which results
in us not seeing the crashing thread, hence you don't get a backtrace (in
fact, the backtracer used to crash, since we never expected to be in that
situation).
Instead, use `NtQuerySystemInformation()` to get the thread list, then
`OpenProcess()` and `OpenThread()` to get process/thread handles. This
should be more robust.
rdar://171438432
