Add dwarf2_per_objfile parameter to free_one_cached_comp_unit

Add dwarf2_per_objfile parameter to free_one_cached_comp_unit

This allows removing some references to
dwarf2_per_cu_data::dwarf2_per_objfile.

gdb/ChangeLog:

	* dwarf2/read.h (struct dwarf2_queue_item): Add
	dwarf2_per_objfile parameter, assign new parameter.
	<per_objfile>: New field.
	* dwarf2/read.c (free_one_cached_comp_unit): Add
	dwarf2_per_objfile parameter.
	(queue_comp_unit): Likewise.
	(dw2_do_instantiate_symtab): Update.
	(process_psymtab_comp_unit): Update.
	(maybe_queue_comp_unit): Add dwarf2_per_objfile parameter.
	(process_imported_unit_die): Update.
	(queue_and_load_dwo_tu): Update.
	(follow_die_offset): Update.
	(follow_die_sig_1): Update.

Change-Id: Ibb4a4ea28eeac5ebcbf73c0d2a13f9391e15c235