warnings: raise warning for useless printers or destructors

warnings: raise warning for useless printers or destructors

* src/scan-code.h (code_props): Add field 'is_used'.
(CODE_PROPS_NONE_INIT): Adjust.
* src/scan-code.l (code_props_plain_init, code_props_symbol_action_init)
(code_props_rule_action_init): Instead of implementing several
times the initialization of the code_props structures,
use code_props_none_init.
* src/symtab.c (symbol_check_defined): If a symbol does not have a
destructor (resp. printer) but has a type which has a destructor (resp.
printer), then set field 'is_used' to true.
(semantic_type_check_defined): If a type has a destructor (resp. printer)
but all symbols of this type have already a destructor (resp. printer),
then raise a warning.
* tests/input.at (Useless printers or destructors): New.

Signed-off-by: Akim Demaille <akim@lrde.epita.fr>