Add dwarf2_per_bfd field to dwarf2_per_cu_data

Add dwarf2_per_bfd field to dwarf2_per_cu_data

Some code using dwarf2_per_cu_data objects accesses the corresponding
dwarf2_per_bfd using the following pattern:

    per_cu->dwarf2_per_objfile->per_bfd

Since dwarf2_per_cu_data objects are going to become
objfile-independent, the dwarf2_per_objfile link must go.  To replace
it, add a dwarf2_per_cu_data->per_bfd link.  It makes sense to have it
there because the dwarf2_per_cu_data objects belong to the
dwarf2_per_bfd, so this is essentially just a backlink to their owner.

gdb/ChangeLog:

	* dwarf2/read.h (struct dwarf2_per_cu_data) <per_bfd>: New
	member.
	* dwarf2/read.c (dwarf2_per_bfd::allocate_per_cu): Initialize
	dwarf2_per_cu_data::per_bfd.
	(dwarf2_per_bfd::allocate_signatured_type): Likewise.
	(create_type_unit_group): Likewise.
	(queue_comp_unit): Remove reference to
	per_cu->dwarf2_per_objfile.
	(maybe_queue_comp_unit): Likewise.
	(fill_in_sig_entry_from_dwo_entry): Assign new field.
	(create_cus_hash_table): Assign new field.

Change-Id: I4ba0a393e64a14489ef061261a3dede1509d055b