Async-streams: await foreach doesn't dispose result from ConfigureAwait

[jcouv]

ConfiguredCancelableAsyncEnumerable<T>.Enumerator.DisposeAsync() returns ConfiguredValueTaskAwaitable, which doesn't match the interface.

The problem is a side-effect of the recent decision to restrict pattern-based disposal to ref structs. Maybe we need to relax that restriction for async case.

Note: when we implement pattern-based disposal in await foreach (on any type), then we should also implement pattern-based disposal in await using (statements or declarations, also on any type).

Relates to API https://github.com/dotnet/corefx/issues/33909

Umbrella for async-streams feature: #24037

[jcouv]

From discussion in LDM today:

• we want to allow pattern-based disposal in await foreach and await using (statement or declaration)
• extension methods would not contribute
• when we bound to a pattern-based DisposeAsync() method, we won't emit the enumerator as IAsyncDisposable check

[jcouv]

FYI @stephentoub The fix for pattern-based disposal is merged into dev16 preview3.