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.

Change-Id: I1e1c1870820aec23701edc9c3994612da5781c23