[DebugInfo] Make InstrRange into a class, NFC
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.
Differential Revision: https://reviews.llvm.org/D59938