Forbid use of <?= as a semi-reserved identifier

Forbid use of <?= as a semi-reserved identifier

One of the weirdest pieces of PHP code I've ever seen. In terms
of tokens, this gets internally translated to

    use x as y; echo as my_echo;

On master it crashes because this "echo" does not have attached
identifier metadata. Make sure it is added and then reject the
use of "<?=" as an identifier inside zend_lex_tstring.

Fixes oss-fuzz #23547.