[6.0][Concurrency] Consider isolated key-path components when computing the required isolation of a stored property initializer.

[hborla]

• Explanation: Stored property initializers are allowed to be isolated to a global actor if the stored property itself shares the same isolation. The code for computing the required isolation of such an initializer did not consider isolated key-paths, which can only be formed within that isolation domain. This change fixes that, and allows isolated key-path components to be used in stored property initializers when the stored property itself is isolated to the same global actor:

  @MainActor
  class C {
    let kp: KeyPath<C, Bool> = \.x // okay because 'kp' is '@MainActor'-isolated
    var x: Bool = true
  }

• Scope: Only impacts key-paths with isolated path components in stored property initializers under -strict-concurrency=complete or -swift-version 6.

• Issue: Strict Concurrency warning false positive: MainActor isolated KeyPath #72181

• Risk: Low. The fix uses an existing mechanism for initializer expression isolation that just wasn't called when checking key-path isolation in the actor isolation checker.

• Testing: Added a new tests.

• Reviewer: @ktoso @xedin

• Main branch PR: [Concurrency] Consider isolated key-path components when computing the required isolation of a stored property initializer. #75091

[hborla]

@swift-ci please test