Fixed remote failures in TestCPP11EnumTypes, probably others

Typically, LLGS only sends stdout/stderr notifications when the inferior
process is running.

Because LLGS reads stdout from the process in a separate thread, sometimes
these stdout notifications can be received after the server has sent a thread
stop message. The host isn't expecting stdout to be generated by the target
after a stop message and these messages interfere with the host's request/
response paradigm.

Differential Revision: http://reviews.llvm.org/D9024

llvm-svn: 234995