Add std::is_constant_evaluated wrapper around __builtin_is_constant_evaluated

--- libstdc++-v3/include/bits/c++config.jj	2018-08-01 20:14:18.214540951 +0200
+++ libstdc++-v3/include/bits/c++config	2018-12-11 12:23:21.417491141 +0100
@@ -627,6 +627,9 @@  namespace std
 # define _GLIBCXX_HAVE_BUILTIN_HAS_UNIQ_OBJ_REP 1
 # define _GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE 1
 # define _GLIBCXX_HAVE_BUILTIN_LAUNDER 1
+# if __GNUC__ >= 9
+#  define _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED 1
+# endif
 #elif defined(__is_identifier)
 // For non-GNU compilers:
 # if ! __is_identifier(__has_unique_object_representations)
@@ -638,6 +641,9 @@  namespace std
 # if ! __is_identifier(__builtin_launder)
 #  define _GLIBCXX_HAVE_BUILTIN_LAUNDER 1
 # endif
+# if ! __is_identifier(__builtin_is_constant_evaluated)
+#  define _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED 1
+# endif
 #endif // GCC
 
 // End of prewritten config; the settings discovered at configure time follow.
--- libstdc++-v3/include/std/type_traits.jj	2018-11-29 14:39:27.231036940 +0100
+++ libstdc++-v3/include/std/type_traits	2018-12-11 12:24:44.713130332 +0100
@@ -3029,6 +3029,12 @@  template <typename _From, typename _To>
   template<typename _Tp>
     using unwrap_ref_decay_t = typename unwrap_ref_decay<_Tp>::type;
 
+#ifdef _GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED
+  constexpr inline bool
+  is_constant_evaluated() noexcept
+  { return __builtin_is_constant_evaluated(); }
+#endif
+
 #endif // C++2a
 
 _GLIBCXX_END_NAMESPACE_VERSION
--- libstdc++-v3/testsuite/20_util/is_constant_evaluated/1.cc.jj	2018-12-11 10:30:11.964508310 +0100
+++ libstdc++-v3/testsuite/20_util/is_constant_evaluated/1.cc	2018-12-11 12:25:36.495284363 +0100
@@ -0,0 +1,80 @@ 
+// Copyright (C) 2018 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/>.
+
+// { dg-options "-std=gnu++2a" }
+// { dg-do run { target c++2a } }
+
+#include <type_traits>
+#include <testsuite_hooks.h>
+
+template<int N>
+struct X { int v = N; };
+X<std::is_constant_evaluated()> x; // type X<true>
+int y = 4;
+int a = std::is_constant_evaluated() ? y : 1; // initializes a to 1
+int b = std::is_constant_evaluated() ? 2 : y; // initializes b to 2
+int c = y + (std::is_constant_evaluated() ? 2 : y); // initializes c to 2*y
+int d = std::is_constant_evaluated(); // initializes d to 1
+int e = d + std::is_constant_evaluated(); // initializes e to 1 + 0
+
+constexpr int
+foo(int x)
+{
+  const int n = std::is_constant_evaluated() ? 13 : 17; // n == 13
+  int m = std::is_constant_evaluated() ? 13 : 17; // m might be 13 or 17 (see below)
+  char arr[n] = {}; // char[13]
+  return m + sizeof (arr) + x;
+}
+
+constexpr int
+bar()
+{
+  const int n = std::is_constant_evaluated() ? 13 : 17;
+  X<n> x1;
+  X<std::is_constant_evaluated() ? 13 : 17> x2;
+  static_assert(std::is_same<decltype(x1), decltype(x2)>::value,
+		"x1/x2's type");
+  return x1.v + x2.v;
+}
+
+int p = foo(0); // m == 13; initialized to 26
+int q = p + foo(0); // m == 17 for this call; initialized to 56
+static_assert(bar() == 26, "bar");
+
+struct S { int a, b; };
+
+S s = { std::is_constant_evaluated() ? 2 : 3, y };
+S t = { std::is_constant_evaluated() ? 2 : 3, 4 };
+
+static_assert(std::is_same<decltype(x), X<true> >::value, "x's type");
+
+void
+test01()
+{
+  VERIFY( a == 1 && b == 2 && c == 8 && d == 1 && e == 1 && p == 26 );
+  VERIFY( q == 56 && s.a == 3 && s.b == 4 && t.a == 2 && t.b == 4 );
+  VERIFY( foo (y) == 34 );
+  if constexpr (foo (0) != 26)
+    VERIFY( 0 );
+  constexpr int w = foo (0);
+  VERIFY( w == 26 );
+}
+
+int main()
+{
+  test01();
+}
--- libstdc++-v3/testsuite/20_util/is_constant_evaluated/noexcept.cc.jj	2018-12-11 10:48:27.923528892 +0100
+++ libstdc++-v3/testsuite/20_util/is_constant_evaluated/noexcept.cc	2018-12-11 12:25:45.138143167 +0100
@@ -0,0 +1,23 @@ 
+// { dg-options "-std=gnu++2a" }
+// { dg-do compile { target c++2a } }
+
+// Copyright (C) 2018 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 <type_traits>
+
+static_assert(noexcept(std::is_constant_evaluated()));