Fix #3089: Add comment regarding .constraint prefix expressed as cast in C#

ICSharpCode.Decompiler.Tests/TestCases/Pretty/ConstructorInitializers.cs

@@ -60,7 +60,11 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 		public class MethodCallInCtorInit
 		{
 			public MethodCallInCtorInit(ConsoleKey key)
+#if MCS5
+				: this(((int)key/*cast due to .constrained prefix*/).ToString())
+#else
 				: this(((int)key).ToString())
+#endif
 			{
 			}
 

ICSharpCode.Decompiler/CSharp/CallBuilder.cs

@@ -507,6 +507,10 @@ namespace ICSharpCode.Decompiler.CSharp
 					targetExpr = new MemberReferenceExpression(castExpression, methodName);
 					typeArgumentList = ((MemberReferenceExpression)targetExpr).TypeArguments;
 				}
+				if (constrainedTo != null && targetExpr is MemberReferenceExpression { Target: CastExpression cast })
+				{
+					cast.AddChild(new Comment("cast due to .constrained prefix", CommentType.MultiLine), Roles.Comment);
+				}
 			}
 			else
 			{