libstdc++: Use const_iterator in std::set::find<K> return type

François noticed that the "wrong" type is used in the return type for a
std::set member function template.

The iterator for our std::set is the same type as const_iterator,
so this doesn't actually matter. But it's clearer if the return type
matches the type used in the function body.

libstdc++-v3/ChangeLog:

	* include/bits/stl_set.h (set::find): Use const_iterator in
	return type, not iterator.

libstdc++-v3/include/bits/stl_set.h

@@ -875,7 +875,7 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER
       template<typename _Kt>
 	auto
 	upper_bound(const _Kt& __x) const
-	-> decltype(iterator(_M_t._M_upper_bound_tr(__x)))
+	-> decltype(const_iterator(_M_t._M_upper_bound_tr(__x)))
 	{ return const_iterator(_M_t._M_upper_bound_tr(__x)); }
 #endif
       ///@}