But then what is this arg_type &arg?
That again is just there at every level so that the constructors can pass
down the copier or collector reference to something that might need it.
It's only actually used for the line_info_table, and actually just thrown
away in the value_lineptr constructor. I have it there because some vague
ideas about the future support for consolidating and generating the
.debug_line chunks could do some more magic about sharing the file name
strings and that sort of thing. It's just a stub for now.