Java slicing: support -> this/-> paramN contracts

Fixes IDEA-121571 Feature request: A Contract Annotation that indicates that a method returns its argument

java/java-tests/testData/codeInsight/slice/forward/Append.java

@@ -0,0 +1,6 @@
+class Test {
+  StringBuilder <flown111111>builder(StringBuilder <caret>sb) {
+    StringBuilder <flown1111>foo = <flown111><flown11><flown1>sb.append("foo").append(1);
+    return <flown11111>foo;
+  }
+}