[libcxx][test] Update directory_entry test for C++20

P1614R2 removes most of `directory_entry`'s member comparison operators, leaving only `operator==` and `operator<=>`. This test should require the comparison expressions to be valid rather than require the member functions to be present so it is correct in both C++17 and C++20 modes.

GitOrigin-RevId: 2dd0c4d8462658c9a721356b306aa52825612416
1 file changed