This lets us specify all fields in the v5 header. Since v5 entries are
form-based, I've extracted the relevant parts of the debug_info DIE
writing code so it could be reused here as well.
The v5 file and directory entries are more expressive than <=v4 ones, so
one could in theory store everything in the v5 format, while still
reading (YAML) and writing (raw DWARF) in the old format. However, that
would create more corner cases (what if the data cannot be represented
in the older format), and it didn't seem like it was particularly
worthwhile to handle those.
f1fe791ab5