Renamed member function foo in is_polymorphic test code, to prevent warnings when the template argument has a function named "foo".

[SVN r25734]

libs/type_traits

@@ -1 +1 @@
-Subproject commit 3893bcc538fbac97680762ca8d7928beb74dc98a
+Subproject commit 23278c28f2efe56a6b4a2199aa6098db8f39aea7