Fix PR24114 - std::atomic for non-Clang is not a literal type

Add _LIBCPP_CONSTEXPR to the implementation of __gcc_atomic_t.


git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@242172 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/atomic b/include/atomic
index 5bc71f0..97a998d 100644
--- a/include/atomic
+++ b/include/atomic
@@ -554,7 +554,8 @@
 template <typename _Tp>
 struct __gcc_atomic_t {
   __gcc_atomic_t() _NOEXCEPT {}
-  explicit __gcc_atomic_t(_Tp value) _NOEXCEPT : __a_value(value) {}
+  _LIBCPP_CONSTEXPR explicit __gcc_atomic_t(_Tp value) _NOEXCEPT
+    : __a_value(value) {}
   _Tp __a_value;
 };
 #define _Atomic(x) __gcc_atomic::__gcc_atomic_t<x>