When a client asks for a queue pending item's extended backtrace,
hold a strong pointer to that extended backtrace thread in the Process
just like we do for asking a thread's extended backtrace.
Also, give extended backtrace threads an invalid ThreadIndexID number.
We'll still give them valid thread_id's.  Clients who want to know the
original thread's IndexID can call GetExtendedBacktraceOriginatingIndexID().
<rdar://problem/16126034>