Add dwarf2_per_objfile parameter to recursively_compute_inclusions

This allows removing dwarf2_per_cu_data::dwarf2_per_objfile references
in recursively_compute_inclusions and compute_compunit_symtab_includes.

gdb/ChangeLog:

	* dwarf2/read.c (recursively_compute_inclusions): Add
	dwarf2_per_objfile parameter.
	(compute_compunit_symtab_includes): Likewise.
	(process_cu_includes): Update.

Change-Id: I1ee7f8dfc07b39763985e6764e8ce04dcc943ec5