Modify mismatched type error for functions with no return

Fix #50009.

```
error[E0308]: mismatched types
  --> $DIR/coercion-missing-tail-expected-type.rs:3:24
   |
LL | fn plus_one(x: i32) -> i32 { //~ ERROR mismatched types
   |    --------            ^^^ expected i32, found ()
   |    |
   |    this function's body doesn't return
LL |     x + 1;
   |          - help: consider removing this semicolon
   |
   = note: expected type `i32`
              found type `()`
```

instead of

```
error[E0308]: mismatched types
  --> $DIR/coercion-missing-tail-expected-type.rs:3:28
   |
LL |   fn plus_one(x: i32) -> i32 { //~ ERROR mismatched types
   |  ____________________________^
LL | |     x + 1;
   | |          - help: consider removing this semicolon
LL | | }
   | |_^ expected i32, found ()
   |
   = note: expected type `i32`
              found type `()`
```