Determine whether Roslyn should automatically opt `[AsyncMethodBuilder]` methods out of runtime async

[jakobbotsch]

We are manually opting some methods that use [AsyncMethodBuilder] out of runtime async over in dotnet/runtime#128943. It may be unexpected behavior for users that enabling runtime async will start ignoring the custom async method builders, so we should determine if this should be a warning or if Roslyn should just automatically opt these methods out of runtime async.

cc @333fred

[teo-tsirpanis]

In general, I think that opting a method out of runtime async by applying [AsyncMethodBuilder(typeof(Async[Value]TaskMethodBuilder[<>]))] seems better and more clear than having to define an attribute type in source, and applying that.

[333fred]

My preference for this type of thing is generally that you'd get told to opt out with either a warning or an error, but I do think that AsyncMethodBuilder is enough of a signal for us to honor it.