Fixed an issue with the pthread_setspecific() where we weren't NULL-ing out

the thread specific data and were destroying the thread specfic data more
than once.

Also added the ability to ask a lldb::StateType if it is stopped with an
additional paramter of "must_exist" which means that the state must be a
stopped state for a process that still exists. This means that eStateExited
and eStateUnloaded will no longer return true if "must_exist" is set to true.

llvm-svn: 144875