[libc++] Use saturation builtins directly for {add,sub}_sat (llvm#165228)

This doesn't improve performance (except with optimizations disabled),
since the compiler is able to fold our current implementation. However,
it does significantly reduce the amount of code the compiler has to sift
through, reducing compile times a bit.

Author: philnik777

libcxx/include/__numeric/saturation_arithmetic.h

@@ -30,6 +30,9 @@ _LIBCPP_BEGIN_NAMESPACE_STD
 
 template <__signed_or_unsigned_integer _Tp>
 _LIBCPP_HIDE_FROM_ABI constexpr _Tp __add_sat(_Tp __x, _Tp __y) noexcept {
+#  if defined(_LIBCPP_CLANG_VER) && _LIBCPP_CLANG_VER >= 2101
+  return __builtin_elementwise_add_sat(__x, __y);
+#  else
   if (_Tp __sum; !__builtin_add_overflow(__x, __y, std::addressof(__sum)))
     return __sum;
   // Handle overflow
@@ -44,10 +47,14 @@ _LIBCPP_HIDE_FROM_ABI constexpr _Tp __add_sat(_Tp __x, _Tp __y) noexcept {
       // Overflows if  (x < 0 && y < 0)
       return std::numeric_limits<_Tp>::min();
   }
+#  endif
 }
 
 template <__signed_or_unsigned_integer _Tp>
 _LIBCPP_HIDE_FROM_ABI constexpr _Tp __sub_sat(_Tp __x, _Tp __y) noexcept {
+#  if defined(_LIBCPP_CLANG_VER) && _LIBCPP_CLANG_VER >= 2101
+  return __builtin_elementwise_sub_sat(__x, __y);
+#  else
   if (_Tp __sub; !__builtin_sub_overflow(__x, __y, std::addressof(__sub)))
     return __sub;
   // Handle overflow
@@ -63,6 +70,7 @@ _LIBCPP_HIDE_FROM_ABI constexpr _Tp __sub_sat(_Tp __x, _Tp __y) noexcept {
       // Overflows if (x < 0 && y > 0)
       return std::numeric_limits<_Tp>::min();
   }
+#  endif
 }
 
 template <__signed_or_unsigned_integer _Tp>