Provide structured suggestion for type mismatch in loop

We currently provide only a `help` message, this PR introduces the last
two structured suggestions instead:

```
error[E0308]: mismatched types
  --> $DIR/issue-98982.rs:2:5
   |
LL |   fn foo() -> i32 {
   |               --- expected `i32` because of return type
LL | /     for i in 0..0 {
LL | |         return i;
LL | |     }
   | |_____^ expected `i32`, found `()`
   |
note: the function expects a value to always be returned, but loops might run zero times
  --> $DIR/issue-98982.rs:2:5
   |
LL |     for i in 0..0 {
   |     ^^^^^^^^^^^^^ this might have zero elements to iterate on
LL |         return i;
   |         -------- if the loop doesn't execute, this value would never get returned
help: return a value for the case when the loop has zero elements to iterate on
   |
LL ~     }
LL ~     /* `i32` value */
   |
help: otherwise consider changing the return type to account for that possibility
   |
LL ~ fn foo() -> Option<i32> {
LL |     for i in 0..0 {
LL ~         return Some(i);
LL ~     }
LL ~     None
   |
```

Fix #98982.

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