Don't print expression result for dbg!() when expr is literal

```rust
fn main() {
    dbg!("Hello, world!");
}
```

Should give:

```
[src/main.rs:2] "Hello, world!"
```

Instead of:

```
[src/main.rs:2] "Hello, world!" = "Hello, world!"
```

This should be possible by adding a macro rule matching `$val:literal` before the rule matching `$val:expr`.