[RFA/DWARF2] Add DW_AT_GNAT_descriptive_type attribute

[Nick Clifton]

Hi Joel,

> 2009-12-24  Joel Brobecker  <brobecker@adacore.com>
> 
>         * dwarf2.h (enum dwarf_attribute): Add DW_AT_GNAT_descriptive_type.
> 
> Tested by rebuilding GDB.  OK to checkin?

Ok from the binutils side.

Cheers
   Nick