[libc++] Also provide an alignment assumption for vector in C++03 mode (llvm#124839)

There's no reason not to, and it's easy enough to do using enable_if. As
a drive-by change, also add a missing _LIBCPP_NO_CFI attribute on
__add_alignment_assumption.

(cherry picked from commit ccb08b9)

Author: ldionne

libcxx/include/__vector/vector.h

@@ -783,14 +783,18 @@ class _LIBCPP_TEMPLATE_VIS vector {
 
   _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI void __move_assign_alloc(vector&, false_type) _NOEXCEPT {}
 
-  static _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI pointer __add_alignment_assumption(pointer __p) _NOEXCEPT {
-#ifndef _LIBCPP_CXX03_LANG
-    if constexpr (is_pointer<pointer>::value) {
-      if (!__libcpp_is_constant_evaluated()) {
-        return static_cast<pointer>(__builtin_assume_aligned(__p, alignof(decltype(*__p))));
-      }
+  template <class _Ptr = pointer, __enable_if_t<is_pointer<_Ptr>::value, int> = 0>
+  static _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI _LIBCPP_NO_CFI pointer
+  __add_alignment_assumption(_Ptr __p) _NOEXCEPT {
+    if (!__libcpp_is_constant_evaluated()) {
+      return static_cast<pointer>(__builtin_assume_aligned(__p, _LIBCPP_ALIGNOF(decltype(*__p))));
     }
-#endif
+    return __p;
+  }
+
+  template <class _Ptr = pointer, __enable_if_t<!is_pointer<_Ptr>::value, int> = 0>
+  static _LIBCPP_CONSTEXPR_SINCE_CXX20 _LIBCPP_HIDE_FROM_ABI _LIBCPP_NO_CFI pointer
+  __add_alignment_assumption(_Ptr __p) _NOEXCEPT {
     return __p;
   }
 };