[libc++] Remove _LIBCPP_PREFERRED_ALIGNOF

[ldionne]

I looked at https://reviews.llvm.org/D54814 again, and I think we should actually keep this, even if we end up defining alignof as a macro. Sorry for mis-leading you in my previous comment on D126249. I think it's too important to retain the notion that __alignof is the *preferred* alignment of the compiler in the name that we use, not only as a comment.

libcxx/include/__config

 #ifndef _LIBCPP_CXX03_LANG
 # define _LIBCPP_ALIGNOF(_Tp) alignof(_Tp)
 #elif defined(_LIBCPP_COMPILER_CLANG_BASED)
 # define _LIBCPP_ALIGNOF(_Tp) _Alignof(_Tp)
 #else
 # error "We don't know a correct way to implement alignof(T) in C++03 outside of Clang"
 #endif
 
-#define _LIBCPP_PREFERRED_ALIGNOF(_Tp) __alignof(_Tp)
-
 #if defined(_LIBCPP_COMPILER_CLANG_BASED)
 
 #if defined(__APPLE__) && !defined(__i386__) && !defined(__x86_64__) &&       \
     (!defined(__arm__) || __ARM_ARCH_7K__ >= 2)
 #  define _LIBCPP_ABI_ALTERNATE_STRING_LAYOUT
 #endif
 
 #if __has_feature(cxx_alignas)

libcxx/include/type_traits

#ifndef _LIBCPP_CXX03_LANG
     __nat& operator=(const __nat&) = delete;
     ~__nat() = delete;
 #endif
 };
 
 template <class _Tp>
 struct __align_type
 {
-    static const size_t value = _LIBCPP_PREFERRED_ALIGNOF(_Tp);
+    // We want the preferred alignof instead of the minimum, so use __alignof instead
+    static const size_t value = __alignof(_Tp);
     typedef _Tp type;
 };
 
 struct __struct_double {long double __lx;};
 struct __struct_double4 {double __lx[4];};
 
 typedef
     __type_list<__align_type<unsigned char>,

 {
     static const size_t value = _I0 >= _I1 ? __static_max<_I0, _In...>::value :
                                              __static_max<_I1, _In...>::value;
 };
 
 template <size_t _Len, class _Type0, class ..._Types>
 struct aligned_union
 {
-    static const size_t alignment_value = __static_max<_LIBCPP_PREFERRED_ALIGNOF(_Type0),
-                                                       _LIBCPP_PREFERRED_ALIGNOF(_Types)...>::value;
+    // We want the preferred alignof instead of the minimum, so use __alignof instead
+    static const size_t alignment_value = __static_max<__alignof(_Type0),
+                                                       __alignof(_Types)...>::value;
     static const size_t __len = __static_max<_Len, sizeof(_Type0),
                                              sizeof(_Types)...>::value;
     typedef typename aligned_storage<__len, alignment_value>::type type;
 };
 
 #if _LIBCPP_STD_VER > 11
 template <size_t _Len, class ..._Types> using aligned_union_t = typename aligned_union<_Len, _Types...>::type;
 #endif