Avoid exposing functions via ADL.

I'd put the iterators in their own namespace so that they wouldn't pick
up functions in detail via ADL, but I forgot that their template
parameters would cause that to happen anyway. The simplest way to fix
that for now is just to stuff the problematic functions into a
sub-namespace, so that they're no longer exposed.

[SVN r85280]

include/boost/unordered/detail/util.hpp

@@ -28,7 +28,10 @@ namespace boost { namespace unordered { namespace detail {
     struct move_tag {};
     struct empty_emplace {};
 
-    template <class T> inline void ignore_unused_variable_warning(T const&) {}
+    namespace func {
+        template <class T>
+        inline void ignore_unused_variable_warning(T const&) {}
+    }
 
     ////////////////////////////////////////////////////////////////////////////
     // iterator SFINAE