C#: Add some unification and viable callable test cases.

Author: michaelnebel

csharp/ql/test/library-tests/dispatch/ViableCallable.cs

@@ -588,3 +588,33 @@ void Run<T>(T c) where T : I3<T>
         c.M13();
     }
 }
+
+public class C21
+{
+    public interface I
+    {
+        void M();
+    }
+
+    public class A1 : I
+    {
+        public void M() { }
+    }
+
+    public ref struct A2 : I
+    {
+        public void M() { }
+    }
+
+    public void Run1<T>(T t) where T : I
+    {
+        // Viable callable: A1.M() [also reports A2.M(); false positive]
+        t.M();
+    }
+
+    public void Run2<T>(T t) where T : I, allows ref struct
+    {
+        // Viable callable: {A1, A2}.M()
+        t.M();
+    }
+}

csharp/ql/test/library-tests/unification/Unification.cs

@@ -48,3 +48,11 @@ public class NestedC<T12> { }
     Nested<int>.NestedB.NestedC<bool> x5;
     Nested<string>.NestedB.NestedC<decimal> x6;
 }
+
+interface I2 { }
+struct S3 : I2 { }
+ref struct RS : I2 { }
+class C7 : I2 { }
+
+class NormalConstraint<T> where T : I2 { } // False positive: Allows T to be `RS`.
+class NegativeConstraint<T> where T : I2, allows ref struct { }