Debugger: Handle DW_AT_signature.

- Per DWARF4's specification, if a type's complete definition is
  contained in a .debug_types unit, it should be referenced via the
  DW_AT_signature attribute. 4.8 now actually does this rather than
  setting e.g. DW_AT_attribute_origin to a signature ref, and
  consequently we weren't finding said reference any more.

Gets .debug_types section support working again.