Rollup merge of #148465 - Jarcho:for_span, r=davidtwco,Zalathar

Adjust spans into the `for` loops context before creating the new desugaring spans.

When lowering `for` loops, the spans for the `into_iter` call and the `Some` pattern used the span of the provided pattern and head expression. If either of those came from a different `SyntaxContext` this would result in some very strange contexts. e.g.:

```rust
macro_rules! m { ($e:expr) => { { $e } } }
for _ in m!(expr) {}
```

This would result in the `into_iter` call have a context chain of `desugar => m!() => root` which is completely nonsensical; `m!()`  does not have a `for` loop. The `into_iter` call also ends up located at `{ $e }` rather than inside the `for _ in _` part.

This fixes that by walking the spans up to the `for` loop's context first. This will not handle adjusting the location of macro variable expansions (e.g. `for _ in $e`), but this does adjust the context to match the `for` loops.

---

This ended up causing rust-lang/rust-clippy#16008. Clippy should be using a `debug_assert` rather than `unreachable`, but it still results in a bug either way.

Author: Zalathar