[POSIX] Replace assert with llvm_unreachable(). NFCI.

llvm-svn: 318348

Author: dcci

lldb/source/Host/posix/HostThreadPosix.cpp

@@ -43,7 +43,7 @@ Status HostThreadPosix::Cancel() {
   if (IsJoinable()) {
 #ifndef __ANDROID__
 #ifndef __FreeBSD__
-    assert(false && "someone is calling HostThread::Cancel()");
+    llvm_unreachable("someone is calling HostThread::Cancel()");
 #endif
     int err = ::pthread_cancel(m_thread);
     error.SetError(err, eErrorTypePOSIX);