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.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@295561 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed