Create a dwarf_attr_die idiom

Create a dwarf_attr_die idiom

We commonly do a dwarf_attr_integrate followed by dwarf_formref_die,
with no care for the Dwarf_Attribute, so I captured that idiom in an
inline function.

* dwarf_wrappers.h (dwarf_attr_die): New.
* dwarf_wrappers.cxx (dwarf_type_name): Call dwarf_attr_die.
* dwflpp.cxx (dwflpp::print_members): Ditto.
  (dwflpp::find_struct_member): Ditto.
* tapsets.cxx (dwarf_derived_probe::saveargs): Ditto.
  (uprobe_derived_probe::saveargs): Ditto.
  (resolve_tracepoint_arg_type): Ditto.
  (tracepoint_derived_probe::build_args): Ditto.