[lldb] Print correct thread plan in logging code of Thread::ShouldReportRun (#188198)
This code accesses the completed thread plan (even if it's private one).
However, the logging code does not pass `skip_private=false` and instead
accesses only the public completed thread plan. In case there is no
public thread plan, the logging code could also crash.
This is just some minor refactoring that ensures we use the same thread
plan in the logging code.