[DWARF parser] Cleanup code in DWARFDebugLine.

Streamline parsing and dumping line tables:
Prefer composition to multiple inheritance in DWARFDebugLine::ParsingState.
Get rid of the weird concept of "DumpingState" structure.

was:
  DWARFDebugLine::DumpingState state(OS);
  DWARFDebugLine::parseStatementTable(..., state);
now:
  DWARFDebugLine::LineTable LineTable;
  LineTable.parse(...);
  LineTable.dump(OS);

No functionality change.