[6.0][Concurrency] Don't allow erasing global actor isolation when the function value crosses an isolation boundary.

[hborla]

• Explanation: The actor isolation checker allows erasing global actor isolation in function conversions when the conversion happens in the same isolation domain and the destination type is not Sendable under the assumption that the function value will never cross an isolation boundary. This is an important rule, because it allows calling synchronous higher order function APIs with global actor isolated function values as long as you're calling from a global actor isolated context. However, the actor isolation checker was allowing the conversion in cases where the function value crosses an isolation boundary:

  func crossIsolationBoundary(_ closure: () -> Void) async { closure() }
  
  @MainActor func test() async {
    let closure = { @MainActor @Sendable in
      MainActor.assertIsolated()
    }
  
    await crossIsolationBoundary(closure)
  }

  This PR diagnoses the function conversion that loses actor isolation if the function conversion is an argument to a call that crosses an isolation boundary.

  We still need to fix this case under region isolation:

  func crossIsolationBoundary(_ closure: () -> Void) async { closure() }
  
  @MainActor func test() async {
    let closure = { @MainActor @Sendable in
      MainActor.assertIsolated()
    }
  
    let erased: () -> Void = closure
  
    await crossIsolationBoundary(erased)
  }

  Currently, the above code is "fine" because the function conversion itself isn't crossing an isolation boundary, and from the perspective of region analysis, erased is in a disconnected region. I think the way to solve this is to merge erased to the main actor region due to the function conversion that erases main actor isolation.

• Scope: Only impacts isolated-to-non-Sendable function conversions that cross isolation boundaries.

• Risk: Low due to narrow scope.

• Testing: Add a test case.

• Reviewer: @xedin

• Main branch PR: [Concurrency] Don't allow erasing global actor isolation when the function value crosses an isolation boundary. #72607

[hborla]

@swift-ci please test