SyntaxAnnotation.cs

AdhocWorkspace workspace = new AdhocWorkspace();
Project project = workspace.AddProject("SampleProject", LanguageNames.CSharp);

//Attach a syntax annotation to the class declaration
var syntaxAnnotation = new SyntaxAnnotation();
var classDeclaration = SyntaxFactory.ClassDeclaration("MyClass")
	.WithAdditionalAnnotations(syntaxAnnotation);

var compilationUnit = SyntaxFactory.CompilationUnit().AddMembers(classDeclaration);

Document document = project.AddDocument("SampleDocument.cs", compilationUnit);
SemanticModel semanticModel = document.GetSemanticModelAsync().Result;

//Use the annotation on our original node to find the new class declaration
var changedClass = document.GetSyntaxRootAsync().Result.DescendantNodes().OfType<ClassDeclarationSyntax>()
	.Where(n => n.HasAnnotation(syntaxAnnotation)).Single();
var symbol = semanticModel.GetDeclaredSymbol(changedClass);