Prepare @$ in %destructor, but currently don't bind it in the

Prepare @$ in %destructor, but currently don't bind it in the
skeleton, as %location use is not cleaned up yet.
* src/scan-gram.l (handle_dollar, handle_destructor_at)
(handle_action_at): New.
(handle_at, handle_action_dollar, handle_destructor_dollar): Take
a braced_code_t and a location as additional arguments.
(handle_destructor_dollar): Instead of requiring `b4_eval', just
unquote one when outputting `b4_dollar_dollar'.
Adjust callers.
* data/bison.simple (b4_eval): Remove.
(b4_symbol_destructor): Adjust.
* tests/input.at (Invalid @n): Adjust.