Description:
------------
When "yield from" was implemented, we did not consider how it interacts with yield-by-reference. Right now it is simply ignored and "yield from" yields all values by-value.

Likely this is not the expected or intended behavior. Instead "yield from" in a by-reference generator should yield array values and values from by-reference generators by-reference. "yield from" should generate an error for iterators (and generators) that do not support by-reference iteration.

Test script:
---------------
<?php
function &gen(array $array) {
    yield from $array;
}

$array = [1, 2, 3];
foreach (gen($array) as &$v) {
    $v += 1;
}
var_dump($array);

Expected result:
----------------
array(3) {
  [0]=>
  int(2)
  [1]=>
  int(3)
  [2]=>
  int(4)
}


Actual result:
--------------
array(3) {
  [0]=>
  int(1)
  [1]=>
  int(2)
  [2]=>
  int(3)
}