[lldb] [MainLoop] Report errno for failed kevent()

[mgorny]

Summary

Modify the kevent() error reporting to use errno rather than returning
the return value. At least on FreeBSD and NetBSD, kevent() always
returns -1 in case of error, and the actual error is returned via errno.

[labath]

That seems right. Even the macos manpage for kevent says the error is returned through errno.

lldb/trunk/source/Host/common/MainLoop.cpp

Status MainLoop::RunImpl::Poll() {
   unsigned i = 0;
   for (auto &fd : loop.m_read_fds)
     EV_SET(&in_events[i++], fd.first, EVFILT_READ, EV_ADD, 0, 0, 0);
 
   num_events = kevent(loop.m_kqueue, in_events.data(), in_events.size(),
                       out_events, llvm::array_lengthof(out_events), nullptr);
 
   if (num_events < 0)
-    return Status("kevent() failed with error %d\n", num_events);
+    return Status(errno, eErrorTypePOSIX);
   return Status();
 }
 
 void MainLoop::RunImpl::ProcessEvents() {
   assert(num_events >= 0);
   for (int i = 0; i < num_events; ++i) {
     if (loop.m_terminate_request)
       return;