Pass dwarf2_cu to process_full_{comp,type}_unit

Pass dwarf2_cu to process_full_{comp,type}_unit

These two functions work on a dwarf2_cu.  It is currently obtained from
the per_cu->cu link, which we want to remove.  Make them accept the
dwarf2_cu directly as a parameter.  This moves the per_cu->cu references
one level up, but that one will be removed too in a subsequent patch.

gdb/ChangeLog:

	* dwarf2/read.c (process_full_comp_unit,
	process_full_type_unit): Remove per_cu, per_objfile paramters.
	Add dwarf2_cu parameter.
	(process_queue): Update.