Align <atomic> with clang r163964 which disallows const _Atomic types.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@164004 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/atomic b/include/atomic
index 6a200eb..6dffdb2 100644
--- a/include/atomic
+++ b/include/atomic
@@ -555,7 +555,7 @@
 template <class _Tp, bool = is_integral<_Tp>::value && !is_same<_Tp, bool>::value>
 struct __atomic_base  // false
 {
-    _Atomic(_Tp) __a_;
+    mutable _Atomic(_Tp) __a_;
 
     _LIBCPP_INLINE_VISIBILITY
     bool is_lock_free() const volatile _NOEXCEPT