gdb/ctf: editorial renames

Rename a few things to fit a bit more with how we typically name things,
or make the names more accurate.  I think this makes the code easier to
follow for anyone familiar with the GDB codebase (or at least, familiar
with the DWARF reader).  It's not super important, but it did help me
understand better the flow of the reader.

 - struct ctf_dict_info -> struct ctf_per_objfile
 - ctf_per_objfile::dict -> ctf_per_objfile::parent_dict
 - ctf_dict_key -> ctf_per_objfile_key
 - ctf_per_tu_data -> ctf_archive_iter_psymtab_data
 - ctf_archive_iter_psymtab_data::dict -> ctf_archive_iter_psymtab_data::parent_dict
 - of -> objfile
 - arc -> archive
 - In build_ctf_archive_member, ctf -> dict

Change-Id: I40f3cba8e40e4c9d006a96032a4c3856bd762ed5
Approved-By: Tom Tromey <tom@tromey.com>