[libc++] Mark usage of _Atomic with __extension__

An upcoming change in Clang will flag _Atomic as being a C11 extension.
To avoid generating this warning in libc++, this commit marks the only
use of _Atomic with the __extension__ extension, which suppresses such
warnings.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@370796 91177308-0d34-0410-b5e6-96231b3b80d8

include/atomic

diff --git a/include/atomic b/include/atomic
index afb431e..0118ab5 100644
--- a/include/atomic
+++ b/include/atomic
@@ -920,7 +920,7 @@
 #endif // _LIBCPP_CXX03_LANG
   _LIBCPP_CONSTEXPR explicit __cxx_atomic_base_impl(_Tp value) _NOEXCEPT
     : __a_value(value) {}
-  _Atomic(_Tp) __a_value;
+  __extension__ _Atomic(_Tp) __a_value;
 };
 
 #define __cxx_atomic_is_lock_free(__s) __c11_atomic_is_lock_free(__s)