[Bug libstdc++/91486] future::wait_for and shared_timed_mutex::wait_for do not work properly with float duration

[john.salmon at deshaw dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91486

--- Comment #5 from John Salmon <john.salmon at deshaw dot com> ---
C++17 already has the needed helper function:  ceil(duration).  

So just change all instances of:

     __clock_t::now() + __reltime

to

    using __dur = typename __clock_t::duration;
    __clock_t::now() + __chrono_detail::ceil<__dur>(__reltime)

and make the C++17 implementation of ceil(duration) visible in all versions as
__chrono_detail::__ceil.