[Focused re-review] SE-0461: Run nonisolated async functions on the caller's actor by default

[hborla (Holly Borla)]

@Nickolas_Pohilets

Needing both @concurrent and nonisolated for functions in actors and GAITs seems redundant to me. IMO, @concurrent on its own is sufficient to non-ambiguously communicate developer's intentions. I'd prefer @concurrent to imply nonisolated

I agree with this. I felt strongly about @exeuction not implying nonisolated when @execution was the base attribute for both behaviors, but now that they're spelled differently, I can't think of any reason not to have @concurrent on its own be sufficient.