Fix C# AddNameToArgument to support AttributeArguments

src/EditorFeatures/CSharpTest/ChangeSignature/AddParameterTests.cs

@@ -377,6 +377,34 @@ public MyAttribute(int y, byte b, string x)
             await TestChangeSignatureViaCommandAsync(LanguageNames.CSharp, markup, updatedSignature: permutation, expectedUpdatedInvocationDocumentCode: updatedCode);
         }
 
+        [WpfFact, Trait(Traits.Feature, Traits.Features.ChangeSignature)]
+        public async Task AddOptionalParameterWithOmittedCallsiteToAttributeConstructor()
+        {
+            var markup = @"
+[Some(1, 2, 4)]
+class SomeAttribute : System.Attribute
+{
+    public SomeAttribute$$(int a, int b, int y = 4)
+    {
+    }
+}";
+            var permutation = new[] {
+                new AddedParameterOrExistingIndex(0),
+                new AddedParameterOrExistingIndex(1),
+                AddedParameterOrExistingIndex.CreateAdded("int", "x", isRequired: false, defaultValue: "3", isCallsiteOmitted: true),
+                new AddedParameterOrExistingIndex(2)};
+            var updatedCode = @"
+[Some(1, 2, y: 4)]
+class SomeAttribute : System.Attribute
+{
+    public SomeAttribute(int a, int b, int x = 3, int y = 4)
+    {
+    }
+}";
+
+            await TestChangeSignatureViaCommandAsync(LanguageNames.CSharp, markup, updatedSignature: permutation, expectedUpdatedInvocationDocumentCode: updatedCode);
+        }
+
         [WpfFact, Trait(Traits.Feature, Traits.Features.ChangeSignature)]
         public async Task AddAndReorderExtensionMethodParametersAndArguments_StaticCall()
         {

src/Features/CSharp/Portable/ChangeSignature/CSharpChangeSignatureService.cs

@@ -811,7 +811,12 @@ protected override IEnumerable<AbstractFormattingRule> GetFormattingRules(Docume
 
         protected override SyntaxNode AddNameToArgument(SyntaxNode newArgument, string name)
         {
-            return ((ArgumentSyntax)newArgument).WithNameColon(NameColon(name));
+            return newArgument switch
+            {
+                ArgumentSyntax a => a.WithNameColon(NameColon(name)),
+                AttributeArgumentSyntax a => a.WithNameColon(NameColon(name)),
+                _ => throw ExceptionUtilities.UnexpectedValue(newArgument.Kind())
+            };
         }
 
         protected override SyntaxNode CreateExplicitParamsArrayFromIndividualArguments(SeparatedSyntaxList<SyntaxNode> newArguments, int indexInExistingList, IParameterSymbol parameterSymbol)