[lldb-vscode] Implement RestartRequest

This is an optional request, but supporting it makes the experience
better when re-launching a big binary that takes significant time to
parse: instead of tearing down and re-create the whole session we just
need to kill the current process and launch a new one.

Some non-obvious comments that might help review this change:

* After killing the process, we get an "exited" event for it. Because
  the process no longer exists some interesting things can occur that
  manifest as flaky failures if not dealt with:

  - `EventIsProcessEvent` relies on `SBEvent::GetBroadcasterClass`,
    which can crash if the broadcaster is no longer there: the event
    only holds a weak_ptr to its broadcaster, and `GetBroadcasterClass`
    uses it without checking.

    Other `EventIs*` functions look at the flavor of the EventData, so I
    have modified EventIsProcessEvent to do that.

  - We keep the PID of the old process so we can detect its "exited"
    event and not terminate the whole session. But sometimes the
    SBProcess we get from the event won't have a PID, for some reason.

* I have factored out the code to launch a process out to a new
  LaunchProcess function, so it can be used from both `request_launch`
  and `request_restart`.

* The restart_runInTerminal test has the same problem with debug builds
  as the original runInTerminal test: when attaching to the launcher
  instance of lldb-vscode it takes a long time to parse its debug info.
  I have used the same workaround to disable that particular test for
  debug builds.

Differential Revision: https://reviews.llvm.org/D147831