[generator] generate only generatable code. (#73)

Fixes: https://bugzilla.xamarin.com/show_bug.cgi?id=43883

The bug describes the situation where generator attempts to generate code
for non-generatable referenced types from assemblies.

tools/generator/ClassGen.cs

@@ -333,7 +333,7 @@ void GenMethods (StreamWriter sw, string indent, CodeGenerationOptions opt)
 			var overridens = defaultMethods.Where (m => overrides.Where (_ => _.Name == m.Name && _.JniSignature == m.JniSignature)
 				.Any (mm => mm.DeclaringType.GetAllDerivedInterfaces ().Contains (m.DeclaringType)));
 
-			foreach (Method m in Methods.Concat (defaultMethods.Except (overridens))) {
+			foreach (Method m in Methods.Concat (defaultMethods.Except (overridens)).Where (m => m.DeclaringType.IsGeneratable)) {
 				bool virt = m.IsVirtual;
 				m.IsVirtual = !IsFinal && virt;
 				if (m.IsAbstract && !m.IsInterfaceDefaultMethodOverride && !m.IsInterfaceDefaultMethod)

tools/generator/CodeGenerator.cs

@@ -323,7 +323,8 @@ public static void Run (CodeGeneratorOptions options)
 			new NamespaceMapping (gens).Generate (opt, gen_info);
 
 			foreach (IGeneratable gen in gens)
-				gen.Generate (opt, gen_info);
+				if (gen.IsGeneratable)
+					gen.Generate (opt, gen_info);
 
 			ClassGen.GenerateTypeRegistrations (opt, gen_info);
 			ClassGen.GenerateEnumList (gen_info);