Allow foreach loops which exit to act like array_filter

[muglug]

class A {}
class B extends A {}

/**
 * @param A[] $arr
 * @return B[]
 */
function filterAList(array $arr) : array {
  foreach ($arr as $a) {
    if (!$a instanceof B) {
      throw new \Exception("bad");
    }
  }
  return $arr;
}

Expected: no error
Actual: MoreSpecificReturnType issue: https://getpsalm.org/r/bcf8ce6c1d

This is fine:

/**
 * @param A[] $arr
 * @return B[]
 */
function filterAList(array $arr) : array {
  return array_filter(
    $arr,
    function(A $a) : bool {
      return $a instanceof B;
    }
  );
}

[muglug]

I've decided not to fix this - it's fairly straightforward to code around it, and there are more common problems still to fix.

[muglug]

For reference, neither Hack nor Typescript nor Flow nor PHPStan understands this type of filtering, so I don't think it's really necessary for Psalm to, either.