<rdar://problem/13700260>
<rdar://problem/13723772>

Modified the lldb_private::Thread to work much better with the OperatingSystem plug-ins. Operating system plug-ins can now return have a "core" key/value pair in each thread dictionary for the OperatingSystemPython plug-ins which allows the core threads to be contained with memory threads. It also allows these memory threads to be stepped, resumed, and controlled just as if they were the actual backing threads themselves.

A few things are introduced:
- lldb_private::Thread now has a GetProtocolID() method which returns the thread protocol ID for a given thread. The protocol ID (Thread::GetProtocolID()) is usually the same as the thread id (Thread::GetID()), but it can differ when a memory thread has its own id, but is backed by an actual API thread.
- Cleaned up the Thread::WillResume() code to do the mandatory parts in Thread::ShouldResume(), and let the thread subclasses override the Thread::WillResume() which is now just a notification.
- Cleaned up ClearStackFrames() implementations so that fewer thread subclasses needed to override them
- Changed the POSIXThread class a bit since it overrode Thread::WillResume(). It is doing the wrong thing by calling "Thread::SetResumeState()" on its own, this shouldn't be done by thread subclasses, but the current code might rely on it so I left it in with a TODO comment with an explanation.