[libc++] Makes saturation functions privately available. (llvm#89503)

These functions are useful in the implementation of the time zone
database. So expose them with private names.

The functions could be exposed before C++ 20, but since libc++ is mostly
C++ 17 complete it seems less useful to allow earlier.

---------

Co-authored-by: Hristo Hristov <zingam@outlook.com>

libcxx/include/__numeric/saturation_arithmetic.h

@@ -25,10 +25,10 @@ _LIBCPP_PUSH_MACROS
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-#if _LIBCPP_STD_VER >= 26
+#if _LIBCPP_STD_VER >= 20
 
 template <__libcpp_integer _Tp>
-_LIBCPP_HIDE_FROM_ABI constexpr _Tp add_sat(_Tp __x, _Tp __y) noexcept {
+_LIBCPP_HIDE_FROM_ABI constexpr _Tp __add_sat(_Tp __x, _Tp __y) noexcept {
   if (_Tp __sum; !__builtin_add_overflow(__x, __y, &__sum))
     return __sum;
   // Handle overflow
@@ -46,7 +46,7 @@ _LIBCPP_HIDE_FROM_ABI constexpr _Tp add_sat(_Tp __x, _Tp __y) noexcept {
 }
 
 template <__libcpp_integer _Tp>
-_LIBCPP_HIDE_FROM_ABI constexpr _Tp sub_sat(_Tp __x, _Tp __y) noexcept {
+_LIBCPP_HIDE_FROM_ABI constexpr _Tp __sub_sat(_Tp __x, _Tp __y) noexcept {
   if (_Tp __sub; !__builtin_sub_overflow(__x, __y, &__sub))
     return __sub;
   // Handle overflow
@@ -65,7 +65,7 @@ _LIBCPP_HIDE_FROM_ABI constexpr _Tp sub_sat(_Tp __x, _Tp __y) noexcept {
 }
 
 template <__libcpp_integer _Tp>
-_LIBCPP_HIDE_FROM_ABI constexpr _Tp mul_sat(_Tp __x, _Tp __y) noexcept {
+_LIBCPP_HIDE_FROM_ABI constexpr _Tp __mul_sat(_Tp __x, _Tp __y) noexcept {
   if (_Tp __mul; !__builtin_mul_overflow(__x, __y, &__mul))
     return __mul;
   // Handle overflow
@@ -81,7 +81,7 @@ _LIBCPP_HIDE_FROM_ABI constexpr _Tp mul_sat(_Tp __x, _Tp __y) noexcept {
 }
 
 template <__libcpp_integer _Tp>
-_LIBCPP_HIDE_FROM_ABI constexpr _Tp div_sat(_Tp __x, _Tp __y) noexcept {
+_LIBCPP_HIDE_FROM_ABI constexpr _Tp __div_sat(_Tp __x, _Tp __y) noexcept {
   _LIBCPP_ASSERT_UNCATEGORIZED(__y != 0, "Division by 0 is undefined");
   if constexpr (__libcpp_unsigned_integer<_Tp>) {
     return __x / __y;
@@ -94,7 +94,7 @@ _LIBCPP_HIDE_FROM_ABI constexpr _Tp div_sat(_Tp __x, _Tp __y) noexcept {
 }
 
 template <__libcpp_integer _Rp, __libcpp_integer _Tp>
-_LIBCPP_HIDE_FROM_ABI constexpr _Rp saturate_cast(_Tp __x) noexcept {
+_LIBCPP_HIDE_FROM_ABI constexpr _Rp __saturate_cast(_Tp __x) noexcept {
   // Saturation is impossible edge case when ((min _Rp) < (min _Tp) && (max _Rp) > (max _Tp)) and it is expected to be
   // optimized out by the compiler.
 
@@ -107,6 +107,35 @@ _LIBCPP_HIDE_FROM_ABI constexpr _Rp saturate_cast(_Tp __x) noexcept {
   return static_cast<_Rp>(__x);
 }
 
+#endif // _LIBCPP_STD_VER >= 20
+
+#if _LIBCPP_STD_VER >= 26
+
+template <__libcpp_integer _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr _Tp add_sat(_Tp __x, _Tp __y) noexcept {
+  return std::__add_sat(__x, __y);
+}
+
+template <__libcpp_integer _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr _Tp sub_sat(_Tp __x, _Tp __y) noexcept {
+  return std::__sub_sat(__x, __y);
+}
+
+template <__libcpp_integer _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr _Tp mul_sat(_Tp __x, _Tp __y) noexcept {
+  return std::__mul_sat(__x, __y);
+}
+
+template <__libcpp_integer _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr _Tp div_sat(_Tp __x, _Tp __y) noexcept {
+  return std::__div_sat(__x, __y);
+}
+
+template <__libcpp_integer _Rp, __libcpp_integer _Tp>
+_LIBCPP_HIDE_FROM_ABI constexpr _Rp saturate_cast(_Tp __x) noexcept {
+  return std::__saturate_cast<_Rp>(__x);
+}
+
 #endif // _LIBCPP_STD_VER >= 26
 
 _LIBCPP_END_NAMESPACE_STD