Replace use of std::pair by creating a class for the debug value
instruction ranges instead. This is a preparatory refactoring for
improving handling of clobbered fragments.
In an upcoming commit the Begin pointer will become a PointerIntPair, so
it will be cleaner to have a getter for that.
I think the proper terminology for this may be isOpen / is Closed? https://en.wikipedia.org/wiki/Interval_(mathematics)