Fix by renaming enumerants that happen to be keywords.

This fixes e.g. the compilation of capnp/schema.capnp.

capnpc-csharp/Model/Enumerant.cs

@@ -2,8 +2,12 @@
 {
     class Enumerant
     {
+        string _literal;
         public TypeDefinition TypeDefinition { get; set; }
-        public string Literal { get; set; }
+        public string Literal {
+            get => _literal;
+            set => _literal = IdentifierRenamer.ToNonKeyword(value);
+        }
         public ushort? Ordinal { get; set; }
         public int CodeOrder { get; set; }
     }

capnpc-csharp/Model/IdentifierRenamer.cs

@@ -0,0 +1,22 @@
+using Microsoft.CodeAnalysis.CSharp;
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace CapnpC.Model
+{
+    public class IdentifierRenamer
+    {
+        public static bool IsAnyKeyword(string str)
+        {
+            return SyntaxFacts.GetKeywordKind(str) != SyntaxKind.None
+                 || SyntaxFacts.GetContextualKeywordKind(str) != SyntaxKind.None;
+        }
+        public static string ToNonKeyword(string str)
+        {
+            // Capnp schema identifiers should be already valid, but could be a keyword
+            if (IsAnyKeyword(str)) return $"@{str}";
+            return str;
+        }
+    }
+}