<atomic>: Remove _ENABLE_ATOMIC_ALIGNMENT_FIX (#723)

Fixes #717.

stl/inc/atomic

@@ -1529,16 +1529,6 @@ public:
         return _Result;
     }
 
-#ifndef _ENABLE_ATOMIC_ALIGNMENT_FIX
-    static_assert(alignof(_Ty) >= sizeof(_Ty) || (sizeof(_Ty) != 2 && sizeof(_Ty) != 4 && sizeof(_Ty) != 8),
-        "You've instantiated std::atomic<T> with sizeof(T) equal to 2/4/8 and alignof(T) < sizeof(T). "
-        "Before VS 2015 Update 2, this would have misbehaved at runtime. "
-        "VS 2015 Update 2 was fixed to handle this correctly, "
-        "but the fix inherently changes layout and breaks binary compatibility. "
-        "Please define _ENABLE_ATOMIC_ALIGNMENT_FIX to acknowledge that you understand this, "
-        "and that everything you're linking has been compiled with VS 2015 Update 2 (or later).");
-#endif // _ENABLE_ATOMIC_ALIGNMENT_FIX
-
 #else // ^^^ don't break ABI / break ABI vvv
 
     _NODISCARD bool is_lock_free() const volatile noexcept {

tests/std/tests/Dev11_0863628_atomic_compare_exchange/test.cpp

@@ -6,8 +6,6 @@
 // Regress\intrin\atomic.cpp
 //
 
-#define _ENABLE_ATOMIC_ALIGNMENT_FIX
-
 #include <assert.h>
 #include <atomic>
 #include <limits>

tests/std/tests/P0024R2_parallel_algorithms_is_sorted/test.cpp

@@ -1,8 +1,6 @@
 // Copyright (c) Microsoft Corporation.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
-#define _ENABLE_ATOMIC_ALIGNMENT_FIX
-
 #include <algorithm>
 #include <assert.h>
 #include <deque>

tests/tr1/tests/atomic/test.cpp

@@ -4,7 +4,6 @@
 // test <atomic> C++11 header
 #define TEST_NAME "<atomic>"
 
-#define _ENABLE_ATOMIC_ALIGNMENT_FIX
 #define _SILENCE_CXX20_ATOMIC_INIT_DEPRECATION_WARNING
 
 #include "tdefs.h"