Yarn: Add stubs for MSVC source-code annotations

src/Yarn/SAL.hpp

+// Copyright 2019 The SwiftShader Authors. All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Stubs SAL annotation macros for platforms that do not support them.
+// See https://docs.microsoft.com/en-us/visualstudio/code-quality/annotating-locking-behavior?view=vs-2019
+
+#ifndef yarn_sal_hpp
+#define yarn_sal_hpp
+
+#ifndef _Requires_lock_held_
+#define _Requires_lock_held_(x)
+#endif
+
+#ifndef _Requires_lock_not_held_
+#define _Requires_lock_not_held_(x)
+#endif
+
+#endif // yarn_sal_hpp