Just an minor cleanup.

Fixes shadow_same's false positive for rust-lang#7915

Fix shadow_same's false positive for async function's params(Fixes rust-lang#7915):

Example Code:
```rust
#![deny(clippy::shadow_same)]

pub async fn foo(_a: i32) {
}
```
Output:
```
error: `_a` is shadowed by itself in `_a
```

Hir:
```rust
pub async fn foo(_a: i32)
 ->
     /*impl Trait*/ #[lang = "from_generator"](move |mut _task_context|
                                                   {
                                                       let _a = _a;
                                                       { let _t = { }; _t }
                                                   })
```

Skip checking async function's params.

changelog: Fix shadow_same's false positive for async function's params