[Concurrency] Start using default isolation set in a file scope

Infer default actor isolation from `using` declaration in the
file scope and use it to override one that is set by `-default-isolation`
flag.

Author: xedin

lib/Sema/TypeCheckConcurrency.cpp

@@ -5831,8 +5831,14 @@ computeDefaultInferredActorIsolation(ValueDecl *value) {
       return {};
     };
 
+    DefaultIsolation defaultIsolation = ctx.LangOpts.DefaultIsolationBehavior;
+    if (auto *SF = value->getDeclContext()->getParentSourceFile()) {
+      if (auto defaultIsolationInFile = SF->getDefaultIsolation())
+        defaultIsolation = defaultIsolationInFile.value();
+    }
+
     // If we are required to use main actor... just use that.
-    if (ctx.LangOpts.DefaultIsolationBehavior == DefaultIsolation::MainActor)
+    if (defaultIsolation == DefaultIsolation::MainActor)
       if (auto result =
               globalActorHelper(ctx.getMainActorType()->mapTypeOutOfContext()))
         return *result;

test/Concurrency/default_isolation.swift

@@ -0,0 +1,43 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+
+// REQUIRES: concurrency
+// REQUIRES: swift_feature_DefaultIsolationPerFile
+
+// RUN: %target-swift-frontend -enable-experimental-feature DefaultIsolationPerFile -emit-sil -swift-version 6 -disable-availability-checking %t/main.swift %t/concurrent.swift | %FileCheck %s
+
+//--- main.swift
+
+using @MainActor
+
+class C {
+  // CHECK: // static C.shared.getter
+  // CHECK-NEXT: // Isolation: global_actor. type: MainActor
+  static let shared = C()
+
+  // CHECK: // C.init()
+  // CHECK-NEXT: // Isolation: global_actor. type: MainActor
+  init() {}
+}
+
+// CHECK: // test()
+// CHECK-NEXT: // Isolation: global_actor. type: MainActor
+func test() {
+  // CHECK: // closure #1 in test()
+  // CHECK-NEXT: // Isolation: nonisolated
+  Task.detached {
+    let s = S(value: 0)
+  }
+}
+
+//--- concurrent.swift
+
+using nonisolated
+
+// CHECK: // S.init(value:)
+// CHECK-NEXT: // Isolation: unspecified
+struct S {
+  // CHECK: // S.value.getter
+  // CHECK-NEXT: // Isolation: unspecified
+  var value: Int
+}

test/Parse/using.swift

@@ -5,16 +5,24 @@
 // REQUIRES: concurrency
 
 using @MainActor
+// expected-note@-1 {{default isolation was previously declared here}}
+
 using nonisolated
+// expected-error@-1 {{invalid redeclaration of file-level default actor isolation}}
 
 using @Test // expected-error {{'using' declaration does not support 'Test' attribute}}
 using test // expected-error {{'using' declaration does not support 'test' modifier}}
 
-using
-@MainActor
+do {
+  using // expected-warning {{expression of type 'Int' is unused}}
+  @MainActor
+// expected-error@+1 {{expected declaration}}
+}
 
-using
-nonisolated
+do {
+  using // expected-warning {{expression of type 'Int' is unused}}
+  nonisolated // expected-error {{cannot find 'nonisolated' in scope}}
+}
 
 do {
   func