Pass dwarf2_per_bfd instead of dwarf2_per_objfile to some index-related functions

Pass dwarf2_per_bfd instead of dwarf2_per_objfile to some index-related functions

All these functions actually only need to receive a dwarf2_per_bfd, pass
that instead of dwarf2_per_objfile.

gdb/ChangeLog:

	* dwarf2/read.c (create_cu_from_index_list): Replace
	dwarf2_per_objfile parameter with dwarf2_per_bfd.
	(create_cus_from_index_list): Likewise.
	(create_cus_from_index): Likewise.
	(create_signatured_type_table_from_index): Likewise.
	(create_cus_from_debug_names_list): Likewise.
	(create_cus_from_debug_names): Likewise.
	(dwarf2_read_gdb_index): Update.
	(dwarf2_read_debug_names): Update.

Change-Id: I8cd7dc04bf815723a48745e7e9b283663dccc1ac