EnC: support async-iterator methods

[jcouv]

I noticed that there is some EnC for handling iterator and async methods, so there will likely be some handling for async-iterator methods too.

FYI @tmat @ivanbasov @jinujoseph
Let me know what you think in terms of scheduling. If this could be looked at in preview3, then I don't need to block this scenario.
Thanks

Async-streams umbrella: #24037

[jcouv]

Here's the design document for async-iterator methods, await foreach and await using statements:
https://github.com/dotnet/roslyn/blob/master/docs/features/async-streams.md
It includes code snippets for the lowered form of different constructs.

I also have a draft article with more details: https://github.com/jcouv/roslyn/blob/blog-post/async-iterator.md

From discussion with @tmat, a few C# 8 features seem relatively straightforward:

• async-iterators: when we build the sequence of awaits, to make sure that it is not changed, we should also include yield return and yield break, as well as new await constructs (await foreach, await using statement and declaration). Users are prevented from editing those.
• await foreach: very much like a foreach. We need to confirm whether it adds any locals. Users are prevented from editing the declaration part of the await foreach once it is entered.
• await using statement: very much like a using. We need to confirm whether it adds any locals. Users are prevented from editing the declaration part of the await foreach once it is entered.

The one that seems more trouble is the using declaration and await using declaration. I filed a separate issue for those (#32589).

[tmat]

This is now implemented.