while clause: suppress flattening of FLWOR expressions

```xquery
for $max in (1 to 10)
return (
  for $a in 0 to $max
  while $a < 1
  return $a
)
```

…is flattened to…

```xquery
for $max in (1 to 10)
for $a in 0 to $max
while $a < 1
return $a
```

…which is wrong.