C#: Add test cases for lambda parameter modifiers.

csharp/ql/test/library-tests/parameters/LambdaParameterModifiers.cs

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+using System;
+using System.Collections.Generic;
+
+public class LambdaParameterModifiers
+{
+    delegate void MyRef(ref int i1);
+    delegate void MyOut(out int i2);
+    delegate void MyIn(in int i3);
+    delegate void MyRefReadonly(ref readonly int i4);
+
+    delegate void MyScopedRef(scoped ref int i5);
+
+    public void M()
+    {
+        // Explicitly typed lambda parameters with modifiers.
+        var l1 = (ref int x1) => x1;
+        var l2 = (out int x2) => x2 = 0;
+        var l3 = (in int x3) => x3;
+        var l4 = (ref readonly int x4) => x4;
+        var l5 = (scoped ref int x5) => x5;
+        var l6 = (params IEnumerable<int> x6) => x6;
+
+        // Implicitly typed lambda parameters with modifiers.
+        MyRef l7 = (ref i1) => { };
+        MyOut l8 = (out i2) => i2 = 0;
+        MyIn l9 = (in i3) => { };
+        MyRefReadonly l10 = (ref readonly i4) => { };
+        MyScopedRef l11 = (scoped ref i5) => { };
+    }
+}