Switch case single `let` assignments forced into multiple

This seems to be due to a very recent change, as I've been using this for years without this happening. And I don't see a config setting in the documentation to turn it off.

```
enum Foo {
  case bar(a: Int, b: Int)
}

// Existing code like this
switch foo {
case let .bar(a, b): break
}

// Is forced into 
switch foo {
case .bar(let a, let b): break
}
```