unittests: Add missing license info to AvailabilityDomainTests.cpp.

Author: tshortli

unittests/AST/AvailabilityDomainTests.cpp

@@ -1,3 +1,15 @@
+//===--- AvailabilityDomainTests.cpp - Tests for AvailabilityDomain -------===//
+//
+// This source file is part of the Swift.org open source project
+//
+// Copyright (c) 2025 Apple Inc. and the Swift project authors
+// Licensed under Apache License v2.0 with Runtime Library Exception
+//
+// See https://swift.org/LICENSE.txt for license information
+// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
+//
+//===----------------------------------------------------------------------===//
+
 #include "swift/AST/AvailabilityDomain.h"
 #include "gtest/gtest.h"