C#: Add some GOOD dead store of local test examples.

Author: michaelnebel

csharp/ql/test/query-tests/Dead Code/DeadStoreOfLocal/DeadStoreOfLocal.cs

@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Linq;
 
 public class DeadStoreOfLocal
 {
@@ -484,3 +485,16 @@ void Pi()
         Console.WriteLine($"Pi, {pi,align:F3}");
     }
 }
+
+class LinqFields
+{
+    private readonly IEnumerable<int> QueryBackedField =
+        from x1 in new[] { 1, 2, 3 }
+        where x1 > 1
+        select x1; // GOOD: The query range variable is used by the LINQ expression.
+
+    public static readonly IEnumerable<int> StaticQueryBackedField =
+        from x2 in new[] { 1, 2, 3 }
+        where x2 > 1
+        select x2; // GOOD: The query range variable is used by the LINQ expression.
+}