Auto merge of #118847 - eholk:for-await, r=compiler-errors

Add support for `for await` loops

This adds support for `for await` loops. This includes parsing, desugaring in AST->HIR lowering, and adding some support functions to the library.

Given a loop like:
```rust
for await i in iter {
    ...
}
```
this is desugared to something like:
```rust
let mut iter = iter.into_async_iter();
while let Some(i) = loop {
    match core::pin::Pin::new(&mut iter).poll_next(cx) {
        Poll::Ready(i) => break i,
        Poll::Pending => yield,
    }
} {
    ...
}
```

This PR also adds a basic `IntoAsyncIterator` trait. This is partly for symmetry with the way `Iterator` and `IntoIterator` work. The other reason is that for async iterators it's helpful to have a place apart from the data structure being iterated over to store state. `IntoAsyncIterator` gives us a good place to do this.

I've gated this feature behind `async_for_loop` and opened #118898 as the feature tracking issue.

r? `@compiler-errors`

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -1409,7 +1409,7 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
                             | ExprKind::Unary(..)
                             | ExprKind::If(..)
                             | ExprKind::While(..)
-                            | ExprKind::ForLoop(..)
+                            | ExprKind::ForLoop { .. }
                             | ExprKind::Match(..),
                         ..
                     }),