Add dwarf2_per_objfile parameter to create_partial_symtab

This allows removing a dwarf2_per_cu_data::dwarf2_per_objfile reference.

gdb/ChangeLog:

	* dwarf2/read.c (create_partial_symtab): Add dwarf2_per_objfile
	parameter.
	(create_type_unit_group): Update.
	(process_psymtab_comp_unit_reader): Update.
	(build_type_psymtabs_reader): Update.

Change-Id: I72e3a8fce8022943ce6992fb623e05636cd0e3a5