Fix getContextClassName and getParentContextClassName

They both returned the late static bound class, not the context
class. This meant that eg "constant('self::FOO')" was actually
returning what "constant('static::FOO')" should have done.

In addition, we often want the Class*, not its name, so
change them to return Class*. The remaining places that then
read the name from the Class* should be fixed to use the Class*
directly (in a later diff).

Finally, noticed that while "defined()" was recently fixed to
support "static::", "constant()" was not. Pulled out a common
function to find the correct Class*.

hphp/runtime/ext/ext_array.cpp

@@ -622,8 +622,6 @@ static Variant f_hphp_get_iterator(VRefParam iterable, bool isMutable) {
                          CREATE_VECTOR1(iterable));
   }
   if (iterable.isObject()) {
-    CStrRef context = g_vmContext->getContextClassName();
-
     ObjectData *obj = iterable.getObjectData();
     Variant iterator;
     while (obj->instanceof(SystemLib::s_IteratorAggregateClass)) {
@@ -631,6 +629,8 @@ static Variant f_hphp_get_iterator(VRefParam iterable, bool isMutable) {
       if (!iterator.isObject()) break;
       obj = iterator.getObjectData();
     }
+    VM::Class*ctx = g_vmContext->getContextClass();
+    CStrRef context = ctx ? ctx->nameRef() : empty_string;
     if (isMutable) {
       if (obj->instanceof(SystemLib::s_IteratorClass)) {
         throw FatalErrorException("An iterator cannot be used for "