[DebugInfo] Add DwarfDebug& to DwarfFile.

Use the DwarfDebug in one function that previously took it as a
parameter, and lay the foundation for use this for other operations
coming soon.
[DebugInfo] Sink DwarfDebug::addCurrentFnArgument down into DwarfFile.

Variable handling will be sunk into DwarfFile so that abstract variables
and the like can be shared across multiple CUs (to handle cross-CU
inlining, for example).