"error: cannot bind by-move with sub-bindings" when sub-binding is _

The `x@foo` construct cannot be used in a match arm if `foo` contains sub-bindings. This is not surprising. However, when the sub-bindings are actually `_`, this should be allowed as `_` is not a real binding.

Example:

```
match a {
    x@Some(_) => x,
    None => None
}
```

Result:

```
foo.rs:3:8: 3:17 error: cannot bind by-move with sub-bindings
foo.rs:3         x@Some(_) => x,
```
