Preserve brackets around if-lets and skip while-lets
r? `@jieyouxu`
Tracked by #124085
Fresh out of #129466, we have discovered 9 crates that the lint did not successfully migrate because the span of `if let` includes the surrounding brackets `(..)` like the following, which surprised me a bit.
```rust
if (if let .. { .. } else { .. }) {
// ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
// the span somehow includes the surrounding brackets
}
```
There is one crate that failed the migration because some suggestion spans cross the macro expansion boundaries. Surely there is no way to patch them with `match` rewrite. To handle this case, we will instead require all spans to be tested for admissibility as suggestion spans.
Besides, there are 4 false negative cases discovered with desugared-`while let`. We don't need to lint them, because the `else` branch surely contains exactly one statement because the drop order is not changed whatsoever in this case.
```rust
while let Some(value) = droppy().get() {
..
}
// is desugared into
loop {
if let Some(value) = droppy().get() {
..
} else {
break;
// here can be nothing observable in this block
}
}
```
I believe this is the one and only false positive that I have found. I think we have finally nailed all the corner cases this time.
dc1ccc5264