Add dwarf2_per_objfile field to dwarf2_cu

Add dwarf2_per_objfile field to dwarf2_cu

Subsequent patches will make dwarf2_per_cu_data objfile-independent.
This means that the dwarf2_per_cu_data::dwarf2_per_objfile field must
go.

The code using a dwarf2_cu structure currently accesses the current
dwarf2_per_objfile object through dwarf2_cu->per_cu->dwarf2_per_objfile.
Since it's ok for the dwarf2_cu to know about the current objfile (a
dwarf2_cu is always used in the context of a particular objfile), add a
dwarf2_per_objfile field to dwarf2_cu.  Upcoming patches will gradually
remove uses of dwarf2_per_cu_data::dwarf2_per_objfile in favor of
dwarf2_cu::dwarf2_per_objfile, until the former can be removed.

gdb/ChangeLog:

	* dwarf2/read.c (struct dwarf2_cu) <dwarf2_cu>: Add parameter.
	<per_objfile>: New member.
	(class cutu_reader) <init_tu_and_read_dwo_dies>: Add parameter.
	(cutu_reader::init_tu_and_read_dwo_dies): Add parameter, update
	call to dwarf2_cu.
	(cutu_reader::cutu_reader): Update.
	(dwarf2_cu::dwarf2_cu): Add parameter, initialize per_objfile.