libstdc++: Reduce header dependencies in <span>

	* include/std/span: Do not include <tuple> and <utility>.
	(tuple_size, tuple_element): Declare.

From-SVN: r279171

libstdc++-v3/ChangeLog

 2019-12-10  Jonathan Wakely  <jwakely@redhat.com>
 
+	* include/std/span: Do not include <tuple> and <utility>.
+	(tuple_size, tuple_element): Declare.
+
 	* include/bits/iterator_concepts.h (indirect_result_t): Do not apply
 	iter_reference_t to parameter pack.
 	* testsuite/24_iterators/indirect_callable/projected.cc: New test.

libstdc++-v3/include/std/span

@@ -39,8 +39,6 @@
 #if __cplusplus > 201703L
 
 #include <type_traits>
-#include <tuple>
-#include <utility>
 #include <array>
 #include <bits/stl_iterator.h>
 #include <bits/range_access.h>
@@ -451,6 +449,9 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       return __sp[_Index];
     }
 
+  template<typename _Tp> struct tuple_size;
+  template<size_t __i, typename _Tp> struct tuple_element;
+
   template<typename _Type, size_t _Extent>
     struct tuple_size<span<_Type, _Extent>>
     : public integral_constant<size_t, _Extent>