Confusing diagnostic when local type transitively captures local var

The following gives a confusing diagnostic:

```swift
func foo() {
  var x = 0
  func bar() { // error: closure captures 'x' before it is declared
    _ = x
  }
  struct S {
    func baz() {
      bar()
    }
  }
}
```

The actual issue is that `baz` is calling `bar`, and as such `bar` can't capture local vars.