Use bfd_get_filename instead of objfile_name in lookup_dwo_unit
There should be no functional difference, as objfile_name defers to
bfd_get_filename if objfile::obfd is non-NULL, which should be the case
here. This allows to remove a reference to
dwarf2_per_cu_data::dwarf2_per_objfile.
gdb/ChangeLog:
* dwarf2/read.c (lookup_dwo_unit): Use bfd_get_filename instead
of objfile_name.