As noted in the thread "Question (bug?) about thread tids when lldb loads a core dump." on lldb-dev, lldb is not showing tids for core files.
For FreeBSD the tid is (somewhat unintuitively) found in the pr_pid field of the NT_PRSTATUS note. Collect it when parsing the note and store it in the thread data. I'm not sure how to obtain it on Linux. I'll update the patch if/when I find out -- this patch isn't committable as-is until then.