PR10594: Provide a cached dwarf_getscopes_die

PR10594: Provide a cached dwarf_getscopes_die

This avoids repeated DIE traversal by caching all parents on the first
call, so future calls are just a simple walk up parent links.

* dwflpp.cxx (dwflpp::getscopes_die): New cached function that mimics
  libdw's dwarf_getscopes_die using cached parent links.
  (dwflpp::cache_die_parents): New function to build the parent cache.
  (dwflpp::~dwflpp): Clean up the parent caches.
  (dwflpp::iterate_over_labels): Use the cached getscopes_die.
  (dwflpp::find_variable_and_frame_base): Ditto.
* tapsets.cxx (dwarf_derived_probe::saveargs): Ditto.
  (uprobe_derived_probe::saveargs): Ditto.
  (dwarf_var_expanding_visitor::visit_target_symbol_context): Ditto.