RBI allows duplicate sending in top-level code

Description

Region-based isolation in the Swift 6 language mode allows duplicate sending of a single non-Sendable value in top-level code, causing potential data races.

The issue does not occur in non-top-level code (e.g., within a function body).

Reproduction

class NonSendable { }

actor MyActor {
    let nonSendable: NonSendable
    
    init(_ nonSendable: NonSendable) {
        self.nonSendable = nonSendable
    }
}

func makeActor(_ nonSendable: sending NonSendable) -> MyActor {
    MyActor(nonSendable)
}

let ns = NonSendable()
let _ = makeActor(ns)
let _ = makeActor(ns)  // no data race detected

func f() {
    let ns = NonSendable()
    let _ = makeActor(ns)
//    let _ = makeActor(ns)  // when uncommenting this line, a data race is (correctly) detected
}

Expected behavior

The compiler should detect and diagnose a potential data race for the duplicate sending of ns in the top-level code.

Environment

The issue occurs when Swift 6 language mode is enabled in the latest relased version of Swift (Swift version 6.0.1 (swift-6.0.1-RELEASE)) and in the latest main development snapshot from 2024-09-25 (Swift version 6.1-dev (LLVM 89ccf4b8a46135a, Swift 6a5ae8d5df144dd)).

Additional information

This issue might be related to the behavior observed in this comment to issue #74820, where a similar problem with region-based isolation in top-level code is described.

Start by running the Swift 6 reproduction from the issue and compare the duplicate sending at top level with the equivalent code inside f(). Trace the compiler's region-based isolation handling for top-level code; done means the second makeActor(ns) call is diagnosed as a potential data race without regressing the function-body behavior.