gdb/dwarf: change some functions from "per objfile" to "per bfd"

I noticed that the following functions accept a "dwarf2_per_objfile",
but they can actually accept a less specific "dwarf2_per_bfd".  This
makes it more obvious that the work they do is per BFD and not per
objfile.

 - add_type_unit
 - lookup_dwo_file_slot
 - create_dwo_unit_in_dwp_v1
 - create_dwp_v2_or_v5_section
 - create_dwo_unit_in_dwp_v2
 - create_dwo_unit_in_dwp_v5
 - lookup_dwo_unit_in_dwp

Change-Id: I200cd77850ce0ffa29fc1b9d924056fdce2559f8
Approved-By: Tom Tromey <tom@tromey.com>
1 file changed