PR 68406: Avoid problem with older host compilers

Older compilers require the Key typedef to be expanded for the traversal
function templates.  This patch uses the same approach as we did for
hash_map.

Tested on x86_64-linux-gnu.

gcc/
	PR bootstrap/68406
	* hash-set.h (hash_set::traverse): Expand Key typedef.

From-SVN: r230549

gcc/ChangeLog

+2015-11-18  Richard Sandiford  <richard.sandiford@arm.com>
+
+	PR bootstrap/68406
+	* hash-set.h (hash_set::traverse): Expand Key typedef.
+
 2015-11-18  Martin Liska  <mliska@suse.cz>
 
 	* config/arm/arm.c (arm_valid_target_attribute_p): Finalize

gcc/hash-set.h

@@ -68,7 +68,7 @@ public:
   /* Call the call back on each pair of key and value with the passed in
      arg.  */
 
-  template<typename Arg, bool (*f)(const Key &, Arg)>
+  template<typename Arg, bool (*f)(const typename Traits::value_type &, Arg)>
   void traverse (Arg a) const
     {
       for (typename hash_table<Traits>::iterator iter = m_table.begin ();