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.