[Bug libstdc++/91906] std::timed_mutex::try_lock_until may not wait for timeout to expire when called with user-defined clock

[mac at mcrowe dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=91906

--- Comment #2 from Mike Crowe <mac at mcrowe dot com> ---
v2 series posted at https://gcc.gnu.org/ml/libstdc++/2019-10/msg00055.html .