Reduce the create_function() memory leak size

Summary: refs facebook/hhvm#4250

I don't think there's a need for a static string here, and it reduces the leak for this from 825 bytes/request to 393 bytes/request

```PHP
<?php
$f = create_function('',
'var_dump(<<<EOF
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
aaaaaaaaaaaaaaaaa
'.rand().'/'.rand().'/'.rand().'
EOF
);'
);
$f();
```

Reviewed By: @markw65

Differential Revision: D1695800

Signature: t1:1695800:1416531505:d8aa4aa98212453a0fe2a45327734b65bf30763a