[jnimarshalmethod-gen] Update the types traversal

The previous try to fix
#381 didn't
work. (adf4c48)

The reason is that we don't get the exception when calling
`assembly.DefinedTypes`, but later during traversal of the enumerable.

So instead of using the `DefinedTypes`, use `GetTypes ()` directly as
the exception comes from it. That should hopefuly allow us to handle
it here.

Related reflection sources:
https://github.com/mono/mono/blob/ab3c897d6851ccf75e840d8b767aafa0d0a32d53/mcs/class/corlib/System.Reflection/Assembly.cs#L1023-L1029

Author: radekdoulik

tools/jnimarshalmethod-gen/App.cs

@@ -245,19 +245,21 @@ void CreateMarshalMethodAssembly (string path)
 
 			PrepareTypeMap (ad.MainModule);
 
-			IEnumerable<TypeInfo> types = null;
+			Type[] types = null;
 			try {
-				types = assembly.DefinedTypes;
+				types = assembly.GetTypes ();
 			} catch (ReflectionTypeLoadException e) {
-				types = (IEnumerable<TypeInfo>) e.Types;
+				types = e.Types;
 				foreach (var le in e.LoaderExceptions)
 					Warning ($"Type Load exception{Environment.NewLine}{le}");
 			}
 
-			foreach (var type in types) {
-				if (type == null)
+			foreach (var systemType in types) {
+				if (systemType == null)
 					continue;
 
+				var type = systemType.GetTypeInfo ();
+
 				if (matchType) {
 					var matched = false;