Use a builder instead of ImmutableDictionary in JSON models

This works around JamesNK/Newtonsoft.Json#652 by avoiding the direct use
of immutable types in the serialization process. The result remains
efficient because the builder class caches the last returned immutable
instance, and will continue returning the same instance as long as no
changes are made to the builder.

Fixes DotNetAnalyzers#1359

StyleCop.Analyzers/StyleCop.Analyzers/Settings/ObjectModel/DocumentationSettings.cs

@@ -36,7 +36,7 @@ internal class DocumentationSettings
         /// This is the backing field for the <see cref="Variables"/> property.
         /// </summary>
         [JsonProperty("variables", DefaultValueHandling = DefaultValueHandling.Ignore)]
-        private ImmutableDictionary<string, string> variables;
+        private ImmutableDictionary<string, string>.Builder variables;
 
         /// <summary>
         /// This is the backing field for the <see cref="XmlHeader"/> property.
@@ -82,7 +82,7 @@ protected internal DocumentationSettings()
         {
             this.companyName = DefaultCompanyName;
             this.copyrightText = DefaultCopyrightText;
-            this.variables = ImmutableDictionary<string, string>.Empty;
+            this.variables = ImmutableDictionary<string, string>.Empty.ToBuilder();
             this.xmlHeader = true;
 
             this.documentExposedElements = true;
@@ -138,7 +138,7 @@ public string CopyrightText
         {
             get
             {
-                return this.variables;
+                return this.variables.ToImmutable();
             }
         }