SE-0461: Run nonisolated async functions on the caller's actor by default

[hborla (Holly Borla)]

@CrystDragon

Compared to other warnings, this may seem overwhelming, especially for libraries compiled with "-warnings-as-errors". The author of such a library will have to manually update all its nonisolated async functions, otherwise it will suddenly stop compiling.

Yeah, this is a fair concern. I think there are a few directions we could go in to make this transition less painful:

1. Add a warning group for this diagnostic so that anybody building with -warnings-as-errors can opt out of this specific warning becoming an error.
2. Implement automatic migration tooling for enabling AsyncCallerExecution via [Pitch] Adoption tooling for upcoming features. This would effectively give people a way to opt into such a warning, instead of enabling a warning by default for everybody.
3. Surface implicit @execution(caller) and @execution(concurrent) attributes in SourceKit's cursor info request, so the attributes are made explicit in editor inspection tools like QuickHelp.

I'm completely open to being convinced that a warning on every nonisolated async function is a bad idea, and that the other ideas above are sufficient for easing the transition to the new semantics. I'm sympathetic to the confusion the intermediate state could cause on what an async function means in a project, but there's a real risk of causing too much code churn and annoyance by introducing a warning.