[ASTGen] Generate "nonisolated" decl modifier

lib/ASTGen/Sources/ASTGen/DeclAttrs.swift

@@ -1965,6 +1965,8 @@ extension ASTGenVisitor {
       return self.generateAccessControlAttr(declModifier: node, level: .public)
     case .open:
       return self.generateAccessControlAttr(declModifier: node, level: .open)
+    case .nonisolated:
+      return self.generateNonisolatedAttr(declModifier: node)?.asDeclAttribute
     case .weak, .unowned:
       return self.generateReferenceOwnershipAttr(declModifier: node)?.asDeclAttribute
     default:
@@ -2001,6 +2003,27 @@ extension ASTGenVisitor {
     }
   }
 
+  func generateNonisolatedAttr(declModifier node: DeclModifierSyntax) -> BridgedNonisolatedAttr? {
+    let isUnsafe: Bool
+    switch node.detail?.detail.rawText {
+    case "unsafe":
+      isUnsafe = true
+    case nil:
+      isUnsafe = false
+    case let text?:
+      // TODO: Diagnose
+      _ = text
+      fatalError("invalid argument for nonisolated modifier")
+    }
+
+    return BridgedNonisolatedAttr.createParsed(
+      self.ctx,
+      atLoc: nil,
+      range: self.generateSourceRange(node),
+      isUnsafe: isUnsafe
+    )
+  }
+
   func generateReferenceOwnershipAttr(declModifier node: DeclModifierSyntax) -> BridgedReferenceOwnershipAttr? {
     // 'weak' -> .weak
     // 'weak(<unexpected>)' -> .weak (with diagnostics)

test/ASTGen/attrs.swift

@@ -215,3 +215,12 @@ struct NE : ~Escapable {}
 }
 @lifetime(borrow borrow) func testNameConflict(_ borrow: E) -> NE { NE() }
 @lifetime(result: source) func testTarget(_ result: inout NE, _ source: consuming NE) { result = source }
+
+actor MyActor {
+  nonisolated let constFlag: Bool = false
+  nonisolated(unsafe) var mutableFlag: Bool = false
+}
+func testNonIsolated(actor: MyActor) {
+  _ = actor.constFlag
+  _ = actor.mutableFlag
+}