[libc++] Refactor memory allocation in basic_string (llvm#128423)

This patch introduces a string-internal API to make the allocation and
deallocation the long string simpler. Before this we had a lot of code
duplication, so ensuring that things were actually correct was
non-trivial.

Author: philnik777

libcxx/include/__memory/allocate_at_least.h

@@ -19,26 +19,31 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
+template <class _Pointer, class _SizeT = size_t>
+struct __allocation_result {
+  _Pointer ptr;
+  _SizeT count;
+
+  _LIBCPP_HIDE_FROM_ABI _LIBCPP_CONSTEXPR __allocation_result(_Pointer __ptr, _SizeT __count)
+      : ptr(__ptr), count(__count) {}
+};
+_LIBCPP_CTAD_SUPPORTED_FOR_TYPE(__allocation_result);
+
 #if _LIBCPP_STD_VER >= 23
 
 template <class _Alloc>
 [[nodiscard]] _LIBCPP_HIDE_FROM_ABI constexpr auto __allocate_at_least(_Alloc& __alloc, size_t __n) {
-  return std::allocator_traits<_Alloc>::allocate_at_least(__alloc, __n);
+  auto __res = std::allocator_traits<_Alloc>::allocate_at_least(__alloc, __n);
+  return __allocation_result{__res.ptr, __res.count};
 }
 
 #else
 
-template <class _Pointer>
-struct __allocation_result {
-  _Pointer ptr;
-  size_t count;
-};
-
-template <class _Alloc>
+template <class _Alloc, class _Traits = allocator_traits<_Alloc> >
 [[__nodiscard__]] _LIBCPP_HIDE_FROM_ABI
-_LIBCPP_CONSTEXPR __allocation_result<typename allocator_traits<_Alloc>::pointer>
+_LIBCPP_CONSTEXPR __allocation_result<typename _Traits::pointer, typename _Traits::size_type>
 __allocate_at_least(_Alloc& __alloc, size_t __n) {
-  return {__alloc.allocate(__n), __n};
+  return __allocation_result<typename _Traits::pointer, typename _Traits::size_type>(__alloc.allocate(__n), __n);
 }
 
 #endif // _LIBCPP_STD_VER >= 23