C#: Fix control flow graph for using declaration statements.

csharp/ql/test/library-tests/csharp8/UsingControlFlow.ql

@@ -0,0 +1,10 @@
+import csharp
+import semmle.code.csharp.controlflow.ControlFlowGraph
+
+query predicate edges(ControlFlow::Node node1, ControlFlow::Node node2, string label, string value) {
+  label = "semmle.label" and
+  exists(ControlFlow::SuccessorType t |
+    node2 = node1.getASuccessorByType(t) and value = t.toString()
+  )
+  and node1.getEnclosingCallable().hasName("TestUsingDeclarations")
+}