[libcxx] Simplify rounding of durations in win32 __libcpp_thread_sleep_for

Also fix a comment typo, and remove a superfluous "std::" qualififcation
in __libcpp_semaphore_wait_timed for consistency.

This mirrors what was suggested in review of
1773eec6.

Differential Revision: https://reviews.llvm.org/D98015