thread: implement sleep_for on Windows
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: 756ef344af921d95d562d9e9f9389127a89a6314@291331 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx