When encountering

```rust
let _ = if true {
    Struct
} else {
    foo() // -> Box<dyn Trait>
};
```

if `Struct` implements `Trait`, suggest boxing the then arm tail expression.

Part of rust-lang#102629.

…olving `impl Trait`

When encountering the following

```rust
// run-rustfix
trait Trait {}
struct Struct;
impl Trait for Struct {}
fn foo() -> Box<dyn Trait> {
    Box::new(Struct)
}
fn bar() -> impl Trait {
    Struct
}
fn main() {
    let _ = if true {
        Struct
    } else {
        foo() //~ ERROR E0308
    };
    let _ = if true {
        foo()
    } else {
        Struct //~ ERROR E0308
    };
    let _ = if true {
        Struct
    } else {
        bar() // impl Trait
    };
    let _ = if true {
        bar() // impl Trait
    } else {
        Struct
    };
}
```

suggest boxing both arms

```rust
    let _ = if true {
        Box::new(Struct) as Box<dyn Trait>
    } else {
        Box::new(bar())
    };
    let _ = if true {
        Box::new(bar()) as Box<dyn Trait>
    } else {
        Box::new(Struct)
    };
```