Add isolation argument to functions taking non-sendable async closures.

[grynspan]

This PR adds an isolation parameter to several API functions that take async closures that are not required to be sendable. As well, it adds sending to those closures' return types where appropriate.

This change is necessary in Swift 6 because a non-sendable async closure could be called in the wrong isolation domain otherwise. In particular, if the caller is @MainActor-isolated, the closure will not be called on the main actor and will therefore always hop, and a concurrency error occurs:

@MainActor func f() async {
  await confirmation {
    // this inner closure is not actor-isolated, but can't be sent across
    // isolation domains.
  }
}

withKnownIssue() and confirmation() are affected, as are several async overloads of the internal-but-public __check() function family.

This change is necessary for correctness, but is potentially source-breaking if calling code directly references the modified functions by full name.

Resolves #622.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

@swift-ci test

[grynspan]

@swift-ci test

[grynspan]

@swift-ci test

[macguru]

I'm really not an expert on this, but I just stumbled across @_inheritActorContext in the Closure isolation control proposal, and am wondering what the tradeoff between that and this pull request's solution is.

As far as I understand, the following change should provide the same behavior to clients, with the benefit of not adding an additional argument to the signature:

func confirmation<R>(
    _ comment: Testing.Comment? = nil,
    expectedCount: Int = 1,
    sourceLocation: Testing.SourceLocation = #_sourceLocation,
    @_inheritActorContext _ body: (Testing.Confirmation) async throws -> R
) async rethrows -> R

Then again, @_inheritActorContext is experimental, it's use "strongly discouraged" and there is maybe more flexibility in having an (any Actor)? inside the function bodies…

[grynspan]

@_inheritActorContext is not the "modern" way to propagate actor isolation into a closure, no. :)

[grynspan]

@swift-ci test