threading_support: make __thread_sleep_for be alertable

On Windows, we were using `Sleep` which is not alertable.  This means
that if the thread was used for a user APC or WinProc handling and
thread::sleep was used, we could potentially dead lock.  Use `SleepEx`
with an alertable sleep, resuming until the time has expired if we are
awoken early.

llvm-svn: 295329