[DWARFLinker] Add CompileUnit::getInfo helper that takes a DWARFDie (NFC)

Eliminate the need to go through the DIE index by passing the DIE to
CompileUnit::getInfo directly.

Before:

  unsigned Idx = Unit->getOrigUnit().getDIEIndex(Die);
  CompileUnit::DIEInfo &Info = Unit->getInfo(Idx);

After:

  CompileUnit::DIEInfo &Info = Unit->getInfo(Die);