Fixed a race condition that was sometimes stopping our command line

interpreter from working. The communication read thread could
startup and immediately exit if m_read_thread_enabled was
checked in the thread function before it was set by the
thread that spawns the read thread. Now m_read_thread_enabled is set
to true prior to spawning the read thread to avoid this issue.

Hopefully this will clear up the sporatic failures in our test suite.

llvm-svn: 113947