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
PR: 241137
MFC after: 1 month
X-MFC-With: r353358
390ceeb437