Add History subclasses for Thread, Unwind, RegisterContext.
Still working out some of the details of these classes but 
I wanted to get the overall structure checked in.
<rdar://problem/15314068> 


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@194245 91177308-0d34-0410-b5e6-96231b3b80d8