Add explicit casts to workaround nvcc bug.

nvcc is failing on evaluating this code in a constant expression
without the explicit casts.

nvbug/3145483

libcxx/include/chrono

@@ -2017,7 +2017,8 @@ constexpr weekday operator-(const weekday& __lhs, const days& __rhs) noexcept
 _LIBCUDACXX_INLINE_VISIBILITY
 constexpr days operator-(const weekday& __lhs, const weekday& __rhs) noexcept
 {
-    const int __wdu = __lhs.c_encoding() - __rhs.c_encoding();
+    // casts are required to work around nvcc bug 3145483
+    const int __wdu = static_cast<int>(__lhs.c_encoding())- static_cast<int>(__rhs.c_encoding());
     const int __wk = (__wdu >= 0 ? __wdu : __wdu-6) / 7;
     return days{__wdu - __wk * 7};
 }