[ping*3] add support for dwarf AT_GNU_descriptive_type

[Olivier Hainque]

Hello,

Ping # 3 (+ took the liberty to cc maintainer) for

  http://gcc.gnu.org/ml/gcc-patches/2010-09/msg00148.html

  << The immediate benefit is improved support for Ada in the debugger
  >>

Thanks much in advance for your feedback,

With Kind Regards,

Olivier