Add a new SBThread::GetExtendedBacktraceOriginatingIndexID() method

(and same thing to Thread base class) which can be used when looking
at an ExtendedBacktrace thread; it will try to find the IndexID() of
the original thread that was executing this backtrace when it was
recorded.  If lldb can't find a record of that thread, it will return
the same value as IndexID() for the ExtendedBacktrace thread.

llvm-svn: 194912