Fix a race condition where 2 threads might try to call Process::SetExitStatus() at the same time.

The problem was the mutex was only protecting the setting of m_exit_string and m_exit_string, but this function relies on the m_private_state being set to eStateExited in order to prevent more than 1 client setting the exit status. We want to only allow the first caller to succeed.

On MacOSX we have a thread that reaps the process we are debugging, and we also have a thread that monitors the debugserver process. When a process exists, the ProcessGDBRemote::AsyncThread() would set the exit status to the correct value and then another thread would reap the debugserver process and they would often both end up in Process::SetExitStatus() at the same time. With the mutex at the top we allow all variables to be set and the m_private_state to be set to eStateExited _before_ the other thread (debugserver reaped) can try to set th exist status to -1 and "lost connection to debugserver" being set as the exit status.

This was probably an issue for lldb-server as well and could very well cleanup some tests that might have been expecting a specific exit status from the process being debugged.

llvm-svn: 238794