[RFA 3/3] Use std::vector in gdb_bfd_data

[Tom Tromey]

>>>>> "Simon" == Simon Marchi <simon.marchi@polymtl.ca> writes:
Simon> But instead, can we make the vector a vector of gdb_bfd_ref_ptr, so
Simon> that the default destructor would take care of everything?

Yeah, good idea.  I made this change as well.

Note this can't be done with archive_bfd, because I think the rule is
that members must be closed before the archive is closed.  Though
perhaps we could move the bfd closing into the destructor as well.

Tom