[region-isolation] Add a test showing that we do not infer nonisolated(unsafe) onto named closures.

The semantics we want is for nonisolated(unsafe) to be inferred onto anonymous
closures. We already have that semantics since we do not look through move_value
[var_decl] or begin_borrow [var_decl]. So even though the underlying
partial_apply will be viewed as nonisolated(unsafe), we will not consider that
when determining the nonisolated(unsafe) for the begin_borrow or move_value that
defined the variable whose value is the partial_apply.

Author: gottesmm

test/Concurrency/transfernonsendable_nonisolatedunsafe.swift

@@ -1026,4 +1026,18 @@ func closureTests() async {
     // expected-note @-1 {{Passing value of non-Sendable type '() async -> ()' as a 'sending' argument to static method 'detached(priority:operation:)' risks causing races in between local and caller code}}
     Task.detached { _ = x4a; _ = x4b } // expected-note {{access can happen concurrently}}
   }
+
+  // The reason why this works is that we do not infer nonisolated(unsafe)
+  // passed the begin_borrow [var_decl] of y. So we think the closure is
+  // nonisolated(unsafe), but its uses via the begin_borrow [var_decl] is
+  // not.
+  func testNamedClosure() async {
+    nonisolated(unsafe) let x = NonSendableKlass()
+    let y = {
+      _ = x
+    }
+    sendingClosure(y) // expected-warning {{sending 'y' risks causing data races}}
+    // expected-note @-1 {{'y' used after being passed as a 'sending' parameter}}
+    sendingClosure(y) // expected-note {{access can happen concurrently}}
+  }
 }