DWP parsing: Use the index by hash when available

Rather than looking up by offset - actually use the hash table to
perform faster lookup where possible. (for DWARFv4 DWP compilation units
the hash isn't in the header - it's in the root DIE, but to parse the
DIE you need the abbrev section and to get the abbrev section you need
the index - so in that case lookup by offset is required)