let else: wrong diagnostic span on return type mismatch

The following code [[playground](https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=e63ca5bc5a1fef571b6c0e93c4e47eb2)]

```rust
#![feature(let_else)]

fn foo() -> u32 {
    println!("foo");

    let () = () else {
        return 12;
    };

    println!("foo");
}
```

fails to build with the following:

```
error[E0308]: mismatched types
 --> src/lib.rs:6:5
  |
6 | /     let () = () else {
7 | |         return 12;
8 | |     };
  | |______^ expected `u32`, found `()`
```

note the span points to just the `let .. else`, but there's nothing wrong with it, the problem is the function return type doesn't match. I'd have expected the error to look the same as if the `let else` wasn't there, which is:

```rust
error[E0308]: mismatched types
 --> src/lib.rs:3:13
  |
3 | fn foo() -> u32 {
  |    ---      ^^^ expected `u32`, found `()`
  |    |
  |    implicitly returns `()` as its body has no tail or `return` expression
```

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let else: wrong diagnostic span on return type mismatch #96961

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Uh oh!

let else: wrong diagnostic span on return type mismatch #96961

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