Auto merge of rust-lang#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 rust-lang#118898 as the feature tracking issue.

r? `@compiler-errors`

Author: bors

core/src/async_iter/async_iter.rs

@@ -47,6 +47,7 @@ pub trait AsyncIterator {
     /// Rust's usual rules apply: calls must never cause undefined behavior
     /// (memory corruption, incorrect use of `unsafe` functions, or the like),
     /// regardless of the async iterator's state.
+    #[cfg_attr(not(bootstrap), lang = "async_iterator_poll_next")]
     fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>>;
 
     /// Returns the bounds on the remaining length of the async iterator.