gdb/objfiles: make objfile::sections yield references

gdb/objfiles: make objfile::sections yield references

I wrote this as a preparatory patch while attempting to make
objfile::section_iterator use filtered_iterator.  It turned out not so
easy, so I have put it aside for now.  But now I have this patch, so I
thought I'd send it by itself.

Since the `obj_section *` yielded by the iterator can't be nullptr, I
think it makes sense for the iterator to yield references instead.
Just like you would get if you iterated on an std::vector<obj_section>.

Change-Id: I7bbee50ed52599e64c4f3b06bdbbde597feba9aa