Make `yield;` a valid statement

Relevant to issue #1627

Next step should be make yield statement to expression and
then implement `Generator::send()/next()` etc methods.

I move two case from good to bad:
- hphp/test/zend/good/Zend/tests/generators/clone.php
- hphp/test/zend/good/Zend/tests/generators/errors/resume_running_generator_error.php

They raise fatal errors because of syntax error but not the real reason it should be.
they should be fixed in anthor diff