Wunused warns on qq fresh names

## reproduction steps

```scala
// -Wunused
val q"$_.unapply($unargs)" = c.macroApplication
```

## problem

```scala
macros.scala:25:9: pattern var qq$macro$1 in method unapply is never used; `qq$macro$1@_` suppresses this warning
[error]     val q"$_.unapply($unargs)" = c.macroApplication
[error]         ^
```

## expectation

`@nowarn`