Fix folding bug with string interpolation

ICSharpCode.Decompiler/CSharp/OutputVisitor/CSharpOutputVisitor.cs

@@ -999,13 +999,13 @@ namespace ICSharpCode.Decompiler.CSharp.OutputVisitor
 		{
 			StartNode(interpolation);
 
-			writer.WriteToken(Roles.LBrace, "{");
+			writer.WriteToken(Interpolation.LBrace, "{");
 			interpolation.Expression.AcceptVisitor(this);
 			if (interpolation.Suffix != null) {
 				writer.WriteToken(Roles.Colon, ":");
 				writer.WritePrimitiveValue("", interpolation.Suffix);
 			}
-			writer.WriteToken(Roles.RBrace, "}");
+			writer.WriteToken(Interpolation.RBrace, "}");
 
 			EndNode(interpolation);
 		}

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

@@ -86,8 +86,11 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 	/// </summary>
 	public class Interpolation : InterpolatedStringContent
 	{
+		public static readonly TokenRole LBrace = new TokenRole("{");
+		public static readonly TokenRole RBrace = new TokenRole("}");
+
 		public CSharpTokenNode LBraceToken {
-			get { return GetChildByRole(Roles.LBrace); }
+			get { return GetChildByRole(LBrace); }
 		}
 
 		public Expression Expression {
@@ -98,7 +101,7 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 		public string Suffix { get; }
 
 		public CSharpTokenNode RBraceToken {
-			get { return GetChildByRole(Roles.RBrace); }
+			get { return GetChildByRole(RBrace); }
 		}
 
 		public Interpolation()

ICSharpCode.Decompiler/Output/TextTokenWriter.cs

@@ -214,6 +214,10 @@ namespace ICSharpCode.Decompiler
 		{
 			switch (token) {
 				case "{":
+					if (role != Roles.LBrace) {
+						output.Write("{");
+						break;
+					}
 					if (braceLevelWithinType >= 0 || nodeStack.Peek() is TypeDeclaration)
 						braceLevelWithinType++;
 					if (nodeStack.OfType<BlockStatement>().Count() <= 1 || FoldBraces) {
@@ -223,6 +227,7 @@ namespace ICSharpCode.Decompiler
 					break;
 				case "}":
 					output.Write('}');
+					if (role != Roles.RBrace) break;
 					if (nodeStack.OfType<BlockStatement>().Count() <= 1 || FoldBraces)
 						output.MarkFoldEnd();
 					if (braceLevelWithinType >= 0)