[libc++][chrono] Avoid tautological comparisions.

[Mordante]

Summary

In our implementation the year is always less than or equal to the
class' max(). It's unlikely this ever changes since changing the
year's range will be an ABI break. A static_assert is added as a
guard.

This was reported by @philnik.

libcxx/include/__chrono/year.h

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef _LIBCPP___CHRONO_YEAR_H
 #define _LIBCPP___CHRONO_YEAR_H
 
 #include <__chrono/duration.h>
 #include <__config>
+#include <limits>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)
 #  pragma GCC system_header
 #endif
 
 _LIBCPP_PUSH_MACROS
 #include <__undef_macros>
 

 _LIBCPP_HIDE_FROM_ABI inline constexpr
 year& year::operator+=(const years& __dy) noexcept
 { *this = *this + __dy; return *this; }
 
 _LIBCPP_HIDE_FROM_ABI inline constexpr
 year& year::operator-=(const years& __dy) noexcept
 { *this = *this - __dy; return *this; }
 
-_LIBCPP_HIDE_FROM_ABI inline constexpr
-bool year::ok() const noexcept
-{ return static_cast<int>(min()) <= __y && __y <= static_cast<int>(max()); }
+_LIBCPP_HIDE_FROM_ABI constexpr bool year::ok() const noexcept {
+  static_assert(static_cast<int>(std::numeric_limits<decltype(__y)>::max()) == static_cast<int>(max()));
+  return static_cast<int>(min()) <= __y;
+}
+
 } // namespace chrono
 
 _LIBCPP_END_NAMESPACE_STD
 
 #endif // _LIBCPP_STD_VER > 17
 
 _LIBCPP_POP_MACROS
 
 #endif // _LIBCPP___CHRONO_YEAR_H