This checkin is the first step in making the lldb thread stepping mechanism more accessible from

the user level.  It adds the ability to invent new stepping modes implemented by python classes,
and to view the current thread plan stack and to some extent alter it.

I haven't gotten to documentation or tests yet.  But this should not cause any behavior changes
if you don't use it, so its safe to check it in now and work on it incrementally.

llvm-svn: 218642