Inform users when *not* running dosep.ty multithreaded.

The gain with multithreading is large, but turning it on requires
an environment variable and so is hard for users to discover.  This
gives users a way to discover the feature by printing out a message
when the environment variable is not set.



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@204018 91177308-0d34-0410-b5e6-96231b3b80d8