Really do make sure that last_write_time.pass.cpp still works with old clang

I *did* try to check that such kind of an issue was not introduced
by the rL315874, but clearly i failed to finish verification.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@315876 91177308-0d34-0410-b5e6-96231b3b80d8