[lldb] Print a message when background tasks take a while to complete (#82799)

When terminating the debugger, we wait for all background tasks to
complete. Given that there's no way to interrupt those treads, this can
take a while. When that happens, the debugger appears to hang at exit.

The above situation is unfortunately not uncommon when background
downloading of dSYMs is enabled (`symbols.auto-download background`).
Even when calling dsymForUUID with a reasonable timeout, it can take a
while to complete.

This patch improves the user experience by printing a message from the
driver when it takes more than one (1) second to terminate the debugger.