libstdc++: ranges::iter_move should perform ADL-only lookup (LWG 3247)

	* include/bits/iterator_concepts.h (iter_move): Add declaration to
	prevent unqualified lookup finding a suitable declaration (LWG 3247).

libstdc++-v3/ChangeLog

 2020-02-19  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/bits/iterator_concepts.h (iter_move): Add declaration to
+	prevent unqualified lookup finding a suitable declaration (LWG 3247).
+
 	* include/std/memory_resource (polymorphic_allocator::allocate)
 	(polymorphic_allocator::allocate_object): Change type of exception to
 	bad_array_new_length (LWG 3237).

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

@@ -79,6 +79,8 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   {
     namespace __cust_imove
     {
+      void iter_move();
+
       template<typename _Tp>
 	concept __adl_imove
 	  = (std::__detail::__class_or_enum<remove_reference_t<_Tp>>)