DebugInfo: Implement debug_line.dwo for file names used in type units during -gsplit-dwarf

This removes an attribute (and more importantly, a relocation) from
skeleton type units and removes some unnecessary file names from the
debug_line section that remains in the .o (and linked executable) file.

There's still a few places we could shave off some more space here:

* use compilation dir of the underlying compilation unit (since all the
  type units share that compilation dir - though this would be more
  complicated in LTO cases where they don't (keep a map of compilation
  dir->line table header?))

* Remove some of the unnecessary header fields from the line table since
  they're not needed in this situation (about 12 bytes per table).