7115046: Add AST node for lambda expressions

Add tree nodes for representing lambda expressions and update relevant visitors interfaces

Reviewed-by: jjg

langtools/src/share/classes/com/sun/source/tree/Tree.java

@@ -186,6 +186,11 @@ public interface Tree {
          */
         NEW_CLASS(NewClassTree.class),
 
+        /**
+         * Used for instances of {@link LambdaExpressionTree}.
+         */
+        LAMBDA_EXPRESSION(LambdaExpressionTree.class),
+
         /**
          * Used for instances of {@link ParenthesizedTree}.
          */