This makes the default work as expected, avoiding the issue brought up
in #3169.

Technically this breaks the following invocation:

```swift
if let wrapped = $shared.optional { /* ... */ }
```

And limits the warning to:

```swift
func share<T>(_: Shared<T>?) {}
share($shared.optional)
```

We consider this lookup to be a bug, though, which is already deprecated
in 1.11.0.

Since we encourage folks to migrate by one minor version at a time, we
should only merge this when we plan on releasing 1.12.0.

…nal-member-lookup