gdb/dwarf: add dwarf2_per_bfd::filename and use it where possible
I noticed we quite often use:
bfd_get_filename (per_bfd->obfd)
Add a shortcut for that.
Change-Id: I4e33925a481fd44088386510b01936d38e1d7d38
Approved-By: Andrew Burgess <aburgess@redhat.com>