Reintroduce CallerSensitive (#25326)

Not having it causes a compile error in 'sbt ++2.12.6 clean akka-actor/compile'
on jdk9, though I can't quite explain why: using the jdk8 classpath otherwise
seems to work, and I can't reproduce it in a minimal example yet.

This reverts commit c6735b630b75408b0c8bbdb22dd31f7d144346ef.

akka-actor/src/main/java/sun/reflect/CallerSensitive.java

@@ -0,0 +1,41 @@
+/*
+ * Copyright (c) 2013, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.  Oracle designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Oracle in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+package sun.reflect;
+
+import java.lang.annotation.*;
+import static java.lang.annotation.ElementType.*;
+
+/**
+ * A method annotated @CallerSensitive is sensitive to its calling class,
+ * via {@link sun.reflect.Reflection#getCallerClass Reflection.getCallerClass},
+ * or via some equivalent.
+ *
+ * @author John R. Rose
+ */
+@Retention(RetentionPolicy.RUNTIME)
+@Target({METHOD})
+public @interface CallerSensitive {
+}