thread: implement sleep_for on Windows

Windows does not provide an implementation of `nanosleep`.  Round up the
time duration to the nearest ms and use `Sleep`.  Although this may
over-sleep, there is no hard real-time guarantee on the wake, so
sleeping a bit more is better than under-sleeping as it within the
specification.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@291331 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed