[lldb] Log to system log instead of stderr from Host::SystemLog

Currently, calls to Host::SystemLog print to stderr on all host
platforms except Darwin. This severely limits its value on the command
line, where we don't want to overload the user with log messages. Switch
to using the syslog function on POSIX systems to send messages to the
system logger instead of stdout.

On Darwin systems this sends the log message to os_log, which matches
what we do today. Nevertheless I kept the current implementation that
uses os_log directly as it gives us more freedom.

I'm not sure if there's an equivalent on Windows, so I kept the existing
behavior of logging to stderr.

Author: JDevlieghere

lldb/source/Host/common/Host.cpp

@@ -89,8 +89,17 @@ using namespace lldb;
 using namespace lldb_private;
 
 #if !defined(__APPLE__)
+#if !defined(_WIN32)
+#include <syslog.h>
+void Host::SystemLog(llvm::StringRef message) {
+  openlog("lldb", LOG_CONS | LOG_PID | LOG_NDELAY, LOG_USER);
+  syslog(LOG_INFO, "%s", message.data());
+  closelog();
+}
+#else
 void Host::SystemLog(llvm::StringRef message) { llvm::errs() << message; }
 #endif
+#endif
 
 #if !defined(__APPLE__) && !defined(_WIN32)
 static thread_result_t