Implement the part of C++20 p1032 Misc constexpr bits.

[Smith-Rowland, Edward M]

I'm going to implement p1032 in pieces.  It *is* miscellaneous after all ;-).

Tested on x96_64-linux? OK?

2019-11-09  Edward Smith-Rowland  <3dw...@verizon.net>

	Implement the <array> part of C++20 p1032 Misc constexpr bits.
	* include/std/array (fill, swap): Make constexpr.
	* testsuite/23_containers/array/requirements/constexpr_fill.cc: New.
	* testsuite/23_containers/array/requirements/constexpr_swap.cc: New.

Index: include/std/array
===================================================================
--- include/std/array	(revision 277944)
+++ include/std/array	(working copy)
@@ -112,11 +112,11 @@
       // 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()); }
@@ -288,6 +288,7 @@
 
   // Specialized algorithms.
   template<typename _Tp, std::size_t _Nm>
+    _GLIBCXX20_CONSTEXPR
     inline
 #if __cplusplus > 201402L || !defined(__STRICT_ANSI__) // c++1z or gnu++11
     // Constrained free swap overload, see p0185r1
@@ -295,7 +296,6 @@
       _GLIBCXX_STD_C::__array_traits<_Tp, _Nm>::_Is_swappable::value
     >::type
 #else
-    _GLIBCXX20_CONSTEXPR
     void
 #endif
     swap(array<_Tp, _Nm>& __one, array<_Tp, _Nm>& __two)
Index: testsuite/23_containers/array/requirements/constexpr_fill.cc
===================================================================
--- testsuite/23_containers/array/requirements/constexpr_fill.cc	(nonexistent)
+++ testsuite/23_containers/array/requirements/constexpr_fill.cc	(working copy)
@@ -0,0 +1,36 @@
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+//
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+//
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+#include <array>
+
+constexpr bool
+test_array()
+{
+  auto ok = true;
+
+  std::array<float,3> fa{};
+  fa.fill(3.333f);
+
+  ok = ok && (fa[0] == fa[2]);
+
+  return ok;
+}
+
+static_assert(test_array());
Index: testsuite/23_containers/array/requirements/constexpr_swap.cc
===================================================================
--- testsuite/23_containers/array/requirements/constexpr_swap.cc	(nonexistent)
+++ testsuite/23_containers/array/requirements/constexpr_swap.cc	(working copy)
@@ -0,0 +1,43 @@
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+//
+// Copyright (C) 2019 Free Software Foundation, Inc.
+//
+// This file is part of the GNU ISO C++ Library.  This library is free
+// software; you can redistribute it and/or modify it under the
+// terms of the GNU General Public License as published by the
+// Free Software Foundation; either version 3, or (at your option)
+// any later version.
+//
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License along
+// with this library; see the file COPYING3.  If not see
+// <http://www.gnu.org/licenses/>.
+
+#include <array>
+
+constexpr bool
+test_array()
+{
+  auto ok = true;
+
+  std::array<float,3> fa{{1.1f, 2.2f, 3.3f}};
+
+  std::array<float,3> fb{{4.4f, 5.5f, 6.6f}};
+
+  fb.swap(fa);
+
+  ok = ok && (fa[0] == 4.4f);
+
+  std::swap(fa, fb);
+
+  ok = ok && (fa[0] == 1.1f);
+
+  return ok;
+}
+
+static_assert(test_array());