Fix 'ref readonly' delegates

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

@@ -4,6 +4,9 @@ namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
 {
 	internal class RefLocalsAndReturns
 	{
+		public delegate ref T RefFunc<T>();
+		public delegate ref readonly T ReadOnlyRefFunc<T>();
+
 		public ref struct RefStruct
 		{
 			private int dummy;

ICSharpCode.Decompiler/CSharp/Syntax/TypeSystemAstBuilder.cs

@@ -1440,6 +1440,9 @@ namespace ICSharpCode.Decompiler.CSharp.Syntax
 				decl.AddAnnotation(new TypeResolveResult(d));
 			}
 			decl.ReturnType = ConvertType(invokeMethod.ReturnType);
+			if (invokeMethod.ReturnTypeIsRefReadOnly && decl.ReturnType is ComposedType ct && ct.HasRefSpecifier) {
+				ct.HasReadOnlySpecifier = true;
+			}
 			decl.Name = d.Name;
 			
 			int outerTypeParameterCount = (d.DeclaringTypeDefinition == null) ? 0 : d.DeclaringTypeDefinition.TypeParameterCount;