DebugInfo: Introduce DebugLocList to encapsulate a list of DebugLocEntries and an MC Label to refer to them

This removes the magic-number-esque code creating/retrieving the same
label for a debug_loc entry from two places and removes the last small
piece of reusable logic from emitDebugLoc so that there will be less
duplication when refactoring it into two functions (one for debug_loc,
the other for debug_loc.dwo).