Fix process launch failure on FreeBSD after r365761

Summary:
After rLLDB365761, and with `LLVM_ENABLE_ABI_BREAKING_CHECKS`
enabled, launching any process on FreeBSD crashes lldb with:

```
Expected<T> must be checked before access or destruction.
Expected<T> value was in success state. (Note: Expected<T> values in
success mode must still be checked prior to being destroyed).
```

This is because `m_operation_thread` and `m_monitor_thread` were
wrapped in `llvm::Expected<>`, but this requires the objects to be
correctly initialized before accessing them.

To fix the crashes, use `llvm::Optional<>` for the members (as
indicated by labath), and use local variables to store the return
values of `LaunchThread` and `StartMonitoringChildProcess`.  Then,
only assign to the member variables after checking if the return
values indicated success.

Reviewers: devnexen, emaste, MaskRay, mgorny

Reviewed By: devnexen

Subscribers: jfb, labath, krytarowski, lldb-commits

Differential Revision: https://reviews.llvm.org/D68723