Compute cu_name dynamically

Compute cu_name dynamically

We only need cu_name for errors and verbose messages, so it's a waste to
always construct it in focus_on_cu.  It's now built only as-needed.

* dwflpp.cxx (dwflpp::cu_name): Now a method instead of a data member.
  (dwflpp::focus_on_module): No cu_name to clear now.
  (dwflpp::focus_on_cu): No cu_name to set now.
  (dwflpp::declaration_resolve): Adjust to call cu_name() now.
  (dwflpp::iterate_over_functions): Ditto.
* tapsets.cxx (query_cu): Ditto.