Add backward dataflow edges through fluent function invocations.

This means that much as obj.getA().setB(...) already has a side-effect on `obj`, all three setters in obj.setA(...).setB(...).setC(...) will have a side-effect on `obj`.

java/ql/test/library-tests/dataflow/fluent-methods/Test.java

@@ -0,0 +1,52 @@
+package smowton;
+
+public class Test {
+
+  private String field;
+
+  public Test fluentNoop() {
+    return this;
+  }
+
+  public Test indirectlyFluentNoop() {
+    return this.fluentNoop();
+  }
+
+  public Test fluentSet(String x) {
+    this.field = x;
+    return this;
+  }
+
+  public static Test identity(Test t) {
+    return t;
+  }
+
+  public String get() {
+    return field;
+  }
+
+  public static String source() {
+    return "taint";
+  }
+
+  public static void sink(String s) {}
+
+  public static void test1() {
+    Test t = new Test();
+    t.fluentNoop().fluentSet(source()).fluentNoop();
+    sink(t.get()); // $hasTaintFlow=y
+  }
+
+  public static void test2() {
+    Test t = new Test();
+    Test.identity(t).fluentNoop().fluentSet(source()).fluentNoop();
+    sink(t.get()); // $hasTaintFlow=y
+  }
+
+  public static void test3() {
+    Test t = new Test();
+    t.indirectlyFluentNoop().fluentSet(source()).fluentNoop();
+    sink(t.get()); // $hasTaintFlow=y
+  }
+
+}