Add dwarf2_per_objfile parameter to process_full_{comp,type}_unit

This allows removing the dwarf2_per_cu_data::dwarf2_per_objfile
references in them.

gdb/ChangeLog:

	* dwarf2/read.c (process_full_comp_unit): Add dwarf2_per_objfile
	parameter.
	(process_full_type_unit): Likewise.
	(process_queue): Update.

Change-Id: Ie68baa8cc4bf1f81cc67d4ad13a59881b4c3feb6