DWARF debug info: provide separate version for line info, so future...

From Appendix F (or G, depending on version of DWARF doc)

Most DWARF sections have a version number in the section header. This version number is not tied to the DWARF standard revision numbers, but instead is incremented when incompatible changes to that section are made.

In order to ease the addition of future DWARF versions this MR separates the line-info version from the version for compilation units.

Currently version 5 for line info is the highest implemented. Future sub-classes will keep that default, to avoid accidental changes.