build: enable ABI breaking std::atomic changes on MSVC

The std::atomic implementation in MSVC broke ABI compatibility across a
minor update (VS2015U2).  Permit the ABI breaking change for the use in
the runtime.

Author: compnerd

cmake/modules/AddSwift.cmake

@@ -241,6 +241,8 @@ function(_add_variant_c_compile_flags)
     list(APPEND result "-D_CRT_USE_WINAPI_FAMILY_DESKTOP_APP")
     # TODO(compnerd) handle /MT
     list(APPEND result "-D_DLL")
+    # NOTE: We assume that we are using VS 2015 U2+
+    list(APPEND result "-D_ENABLE_ATOMIC_ALIGNMENT_FIX")
   endif()
 
   if(CFLAGS_ENABLE_ASSERTIONS)