[6.2][Concurrency] Fix a false-positive metatype capture diagnostic.

[hborla]

• Explanation: sending closures that are isolated to the same actor as the enclosing context don't execute concurrently with the context. Concurrency checking for local captures accounts for this, but there was a bug in the strict Sendable metatype logic from SE-0470 that diagnosed non-Sendable metatypes in sending closures that did not execute concurrently. This change fixes that so that the following pattern is safe:

protocol P {}

func acceptClosure(_: () -> Void) {}

@MainActor
func f<T: P>(_: T.Type) {
  acceptClosure {
    Task {
      _ = T.self // okay to capture T.Type in this closure.
    }
  }
}

• Scope: Only impacts strict concurrency diagnostics when using non-Sendable metatypes in sending closures that are isolated.
• Issues: rdar://152185495
• Original PRs: [Concurrency] Fix a false-positive metatype capture diagnostic. #81860
• Risk: Low; the only effect is skipping diagnostics when checking local captures in the actor isolation checker.
• Testing: Added tests.
• Reviewers: @DougGregor

[hborla]

@swift-ci please test