Fix #1059: String "interpolation", invalid cast when emitting "IL with C#"

ICSharpCode.Decompiler/CSharp/OutputVisitor/CSharpOutputVisitor.cs

@@ -985,11 +985,11 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		{
 			StartNode(interpolatedStringExpression);
 
-			writer.WriteToken(Roles.Error, "$\"");
+			writer.WriteToken(InterpolatedStringExpression.OpenQuote, "$\"");
 			foreach (var element in interpolatedStringExpression.Content) {
 				element.AcceptVisitor(this);
 			}
-			writer.WriteToken(Roles.Error, "\"");
+			writer.WriteToken(InterpolatedStringExpression.CloseQuote, "\"");
 			isAfterSpace = false;
 
 			EndNode(interpolatedStringExpression);

ICSharpCode.Decompiler/CSharp/Syntax/Expressions/InterpolatedStringExpression.cs

@@ -7,6 +7,9 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	public class InterpolatedStringExpression : Expression
 	{
+		public static readonly TokenRole OpenQuote = new TokenRole("$\"");
+		public static readonly TokenRole CloseQuote = new TokenRole("\"");
+
 		public AstNodeCollection<InterpolatedStringContent> Content {
 			get { return GetChildrenByRole(InterpolatedStringContent.Role); }
 		}