Enable thread-safe logging.

Thread-safe logging had been disabled because of a deadlock,
possibly due to a lock acquired during a signal handler.

This patch turns thread safe logging back on and also greatly
reduces the scope of the lock, confining it only to the code that
affects the underlying output stream, instead of all the code that
builds up the formatted log message.  this should resolve the
issue surrounding the deadlock.

llvm-svn: 236892