<rdar://problem/11184458>

Found an issue where we might still have shared pointer references to lldb_private::Thread objects where the object itself is not valid and has been removed from the Process. When a thread is removed from a process, it will call Thread::DestroyThread() which well set a boolean member variable which is exposed now via:

bool
Thread::IsValid() const;

We then check the thread validity before handing out a shared pointer.

llvm-svn: 154048