[lldb] Fixed lldb-server crash (TestLogHandler was not thread safe) (#101326)

Host::LaunchProcess() requires to SetMonitorProcessCallback. This
callback is called from the child process monitor thread. We cannot
control this thread anyway. lldb-server may crash if there is a logging
around this callback because TestLogHandler is not thread safe. I faced
this issue debugging 100 simultaneous child processes. Note
StreamLogHandler::Emit() in lldb/source/Utility/Log.cpp already contains
the similar mutex.

Author: slydiman

lldb/tools/lldb-server/LLDBServerUtilities.cpp

@@ -27,11 +27,13 @@ class TestLogHandler : public LogHandler {
       : m_stream_sp(stream_sp) {}
 
   void Emit(llvm::StringRef message) override {
+    std::lock_guard<std::mutex> guard(m_mutex);
     (*m_stream_sp) << message;
     m_stream_sp->flush();
   }
 
 private:
+  std::mutex m_mutex;
   std::shared_ptr<raw_ostream> m_stream_sp;
 };