[lldb] Make sure the process is stopped when computing the symbol context (#135458)

Make sure the process is stopped when computing the symbol context. Both
Adrian and Felipe reported a handful of crashes in GetSymbolContext
called from Statusline::Redraw on the default event thread.

Given that we're handling a StackFrameSP, it's not clear to me how that
could have gotten invalidated, but Jim points out that it doesn't make
sense to compute the symbol context for the frame when the process isn't
stopped.

Depends on  #135455