Revert "threading_support: make __thread_sleep_for be alertable"

This reverts SVN r295329.  Although `__libcpp_thread_sleep_for` should
be alertable, the implementation causes a large regression in the test
suite.  Add a FIXME item there for now to get the test suite in a better
state before attempting to fix that behaviour.

llvm-svn: 295561
diff --git a/libcxx/include/__threading_support b/libcxx/include/__threading_support
index 8b0b2ed..aa94713 100644
--- a/libcxx/include/__threading_support
+++ b/libcxx/include/__threading_support
@@ -589,14 +589,12 @@
 
 void __libcpp_thread_sleep_for(const chrono::nanoseconds& __ns)
 {
-  using namespace _VSTD::chrono;
-
+  using namespace chrono;
   // round-up to the nearest milisecond
   milliseconds __ms =
       duration_cast<milliseconds>(__ns + chrono::nanoseconds(999999));
-  auto start = system_clock::now();
-  while (::SleepEx((__ms - (system_clock::now() - start)).count(),
-                   TRUE) == WAIT_IO_COMPLETION);
+  // FIXME(compnerd) this should be an alertable sleep (WFSO or SleepEx)
+  Sleep(__ms.count());
 }
 
 // Thread Local Storage