Captures of curried function are incorrectly dropped 

## Compiler version

3.6.0

## Minimized example

```Scala
class C
def test(x: C^) =
  val f = () =>
    def foo() =
      x
      ()
    println(s"hey: $x")
    () => foo()
  val _: () -> () ->{x} Unit = f  // should be error, but compiles
  ()
```

## Output

compiles

## Expectation

An error, since the outer lambda also captures `x` in its println statement.
