Detect when if/else is tail expression has mismatched types that could be a boxed trait

[estebank]

Given the following

struct S;
struct Y;

trait Trait {}

impl Trait for S {}
impl Trait for Y {}

fn baz() -> impl Trait {
    if true {
        S
    } else {
        Y
    }
}

we currently emit

error[E0308]: `if` and `else` have incompatible types
  --> src/main.rs:13:9
   |
10 | /     if true {
11 | |         S
   | |         - expected because of this
12 | |     } else {
13 | |         Y
   | |         ^ expected struct `S`, found struct `Y`
14 | |     }
   | |_____- `if` and `else` have incompatible types

We should suggest changing the return type from impl Trait to Box<dyn Trait> and boxing the two branches. I think this is the last missing suggestion we can give involding impl Trait and dyn Trait returns.

Follow up to #68110.