Revert "[libc++] Refactoring __sync_* builtins; NFC (Reland)"

This reverts commit r307595. The commit had some issues that needed
to first be addressed in review.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@307746 91177308-0d34-0410-b5e6-96231b3b80d8

src/locale.cpp

@@ -28,7 +28,6 @@
 #define _CTYPE_DISABLE_MACROS
 #endif
 #include "cwctype"
-#include "__atomic_support"
 #include "__sso_allocator"
 #if defined(_LIBCPP_MSVCRT) || defined(__MINGW32__)
 #include "support/win32/locale_win32.h"
@@ -668,7 +667,7 @@ locale::id::__get()
 void
 locale::id::__init()
 {
-    __id_ = __libcpp_sync_add_and_fetch(&__next_id, 1);
+    __id_ = __sync_add_and_fetch(&__next_id, 1);
 }
 
 // template <> class collate_byname<char>