Add missing write to polyglotForeignClasses for the base case

* The write used to be in classForTraits() but was moved
  to resolvePolyglotForeignClass() to move that code
  behind the @TruffleBoundary for multi-context.

Author: eregon

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/nodes/object/GetForeignObjectClassNode.java

@@ -167,6 +167,7 @@ private PythonManagedClass classForTraits(int traits) {
     private PythonManagedClass resolvePolyglotForeignClass(int traits) {
         PythonBuiltinClass base = getContext().lookupType(PythonBuiltinClassType.ForeignObject);
         if (traits == 0) {
+            getContext().polyglotForeignClasses[traits] = base;
             return base;
         }