libstdc++: Add C++20 P1032 constexpr to _GLIBCXX_DEBUG array

	* include/debug/array (array<>::fill): Add C++20 constexpr.
	(array<>::swap): Likewise.

From-SVN: r278718

libstdc++-v3/ChangeLog

 2019-11-26  François Dumont  <fdumont@gcc.gnu.org>
 
+	* include/debug/array (array<>::fill): Add C++20 constexpr.
+	(array<>::swap): Likewise.
+
 	* include/debug/safe_iterator.h
 	[__cpp_lib_concepts](_Safe_iterator<>::iterator_concept): Define for
 	C++20.

libstdc++-v3/include/debug/array

@@ -80,11 +80,11 @@ namespace __debug
       // No explicit construct/copy/destroy for aggregate type.
 
       // DR 776.
-      void
+      _GLIBCXX20_CONSTEXPR void
       fill(const value_type& __u)
       { std::fill_n(begin(), size(), __u); }
 
-      void
+      _GLIBCXX20_CONSTEXPR void
       swap(array& __other)
       noexcept(_AT_Type::_Is_nothrow_swappable::value)
       { std::swap_ranges(begin(), end(), __other.begin()); }
@@ -282,6 +282,7 @@ namespace __debug
 #endif
 
   template<typename _Tp, std::size_t _Nm>
+    _GLIBCXX20_CONSTEXPR
     inline void
     swap(array<_Tp, _Nm>& __one, array<_Tp, _Nm>& __two)
     noexcept(noexcept(__one.swap(__two)))