C#: Add test for implicit index usage.

Author: michaelnebel

csharp/ql/test/library-tests/index/Index.cs

@@ -0,0 +1,24 @@
+using System;
+
+public class Container
+{
+    public object[] Buffer { get; } = new object[10];
+}
+
+public class TestIndex
+{
+    public void M()
+    {
+        var c = new Container()
+        {
+            Buffer =
+            {
+                [0] = new object(),
+                [1] = new object(),
+                [^1] = new object()
+            }
+        };
+        c.Buffer[4] = new object();
+        c.Buffer[^3] = new object();
+    }
+}

csharp/ql/test/library-tests/index/Index.expected

@@ -0,0 +1,2 @@
+| Index.cs:18:18:18:19 | ^... | Index.cs:18:19:18:19 | 1 |
+| Index.cs:22:18:22:19 | ^... | Index.cs:22:19:22:19 | 3 |

csharp/ql/test/library-tests/index/Index.ql

@@ -0,0 +1,4 @@
+import csharp
+
+from IndexExpr e
+select e, e.getExpr()