[Distributed] NFC: Add a test-case for different async let configurations

test/Distributed/distributed_actor_async_let.swift

@@ -0,0 +1,70 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-distributed -disable-availability-checking
+
+// UNSUPPORTED: back_deploy_concurrency
+// REQUIRES: concurrency
+// REQUIRES: distributed
+
+import _Distributed
+
+distributed actor Philosopher {
+  let philosophy: String
+
+  typealias Transport = AnyActorTransport
+
+  init(transport: AnyActorTransport) {
+    philosophy = "Epistemology"
+  }
+
+  distributed func hi() -> String { "Hi!" }
+
+  func test(other: Philosopher) async throws {
+    // self --------------------------------------------------------------------
+    async let alet = self.hi() // none
+    _ = await alet // Ok - `self.hi()` isn't throwing
+
+    Task {
+      _ = self.hi() // none
+    }
+
+    Task.detached {
+      _ = await self.hi() // async
+    }
+
+    // other -------------------------------------------------------------------
+
+    async let otherLet = other.hi() // hi = async throws because of `other`
+    _ = try await otherLet
+
+    Task {
+      _ = try await other.hi() // hi = async throws
+    }
+
+    Task.detached {
+      _ = try await other.hi() // hi = async throws
+    }
+
+    // known to be local -------------------------------------------------------
+
+    // FIXME(distributed): relies on the "secretly known to be local" hack in typechecking
+    let _: String? = await other.whenLocal { __secretlyKnownToBeLocal in
+      // we're able to get state of what would otherwise be distributed-isolated:
+      __secretlyKnownToBeLocal.philosophy
+    }
+  }
+
+  static func test(iso: isolated Philosopher) async throws {
+    _ = iso.hi() // we're "in the actor" already, since isolated
+
+    // isolated parameter ------------------------------------------------------
+    async let otherLet = iso.hi() // async
+    _ = await otherLet
+
+    Task {
+      _ = await iso.hi() // none
+    }
+
+    Task.detached {
+      _ = await iso.hi() // async
+    }
+  }
+}