Add (void) cast

[TimWolla]

RFC: https://wiki.php.net/rfc/marking_return_value_as_important

Spun out of #17599, since the (void) cast implementation is fully independent of the #[\NoDiscard] implementation and sufficiently complex on its own, allowing for an easier review.

[ndossche]

I guess you could optimize it a bit by not even handling IS_CONSTANT_AST in the first place, i.e. early returning.
I would still keep the IS_CONST handling below though because it generalises.

[TimWolla]

As with the (void)(bool) cast, this feels like needless added complexity for a case that is unlikely to happen in practice and also no longer matters once the file is compiled and OPcached.

[iluuu1994]

This is missing support for AST printing in zend_ast.c.

LGTM otherwise.