[rfa/dwarf] Support for attributes pointing to a different CU

[Jim Blandy]

> @@ -4610,6 +4821,9 @@ read_subrange_type (struct die_info *die
>        low = 1;
>      }
>  
> +  /* FIXME: For variable sized arrays either of these could be
> +     a variable rather than a constant value.  We'll allow it,
> +     but we don't know how to handle it.  */
>    attr = dwarf2_attr (die, DW_AT_lower_bound, cu);
>    if (attr)
>      low = dwarf2_get_attr_constant_value (attr, 0);

This should be committed separately.