Rollup merge of #108427 - y21:for-else-diagnostic, r=compiler-errors

Recover from for-else and while-else

This recovers from attempts at writing for-else or while-else loops, which might help users coming from e.g. Python.
```rs
for _ in 0..0 {
  // ...
} else {
  // ...
}
```
Combined with trying to store it in a let binding, the current diagnostic can be a bit confusing. It mentions let-else and suggests wrapping the loop in parentheses, which the user probably doesn't want. let-else doesn't make sense for `for` and `while` loops, as they are of type `()` (which already is an irrefutable pattern and doesn't need let-else).
<details>
<summary>Current diagnostic</summary>

```rs
error: right curly brace `}` before `else` in a `let...else` statement not allowed
 --> src/main.rs:4:5
  |
4 |     } else {
  |     ^
  |
help: wrap the expression in parentheses
  |
2 ~     let _x = (for _ in 0..0 {
3 |
4 ~     }) else {
  |
```
</details>

Some questions:
- Can the wording for the error message be improved? Would "for...else loops are not allowed" fit better?
- Should we be more "conservative" in case we want to support this in the future (i.e. say "for...else loops are **currently** not allowed/supported")?
- Is there a better way than storing a `&'static str` for the loop type? It is used for substituting the placeholder in the locale file (since it can emit either `for...else` or `while...else`). Maybe there is an enum I could use that I couldn't find

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