Wrap up the new chrono literals in an #ifdef so that old versions of clang don't complain. I'm looking at you, clang 5.0.1

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@344535 91177308-0d34-0410-b5e6-96231b3b80d8

include/chrono

@@ -2668,7 +2668,7 @@ inline namespace literals
         return chrono::duration<long double, nano> (__ns);
     }
 
-#if _LIBCPP_STD_VER > 17
+#if _LIBCPP_STD_VER > 17 && !defined(_LIBCPP_HAS_NO_CXX20_CHRONO_LITERALS)
     constexpr chrono::day operator ""d(unsigned long long __d) noexcept
     {
         return chrono::day(static_cast<unsigned>(__d));