[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
1773eec6928f4e37b377e23b84d7a2a07d0d1d0d.

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

GitOrigin-RevId: 42d653d294a85ad3a5df1ba0827aeb1ecd99ebe3
1 file changed