Make sure that multi-line expressions don't create a default target. We...

Make sure that multi-line expressions don't create a default target. We recently switched to using a built-in m_exe_ctx when running commands in the DoExecute() so that we can have common code where commands can required having a valid target/process/thread/frame by specifying flags, this caused multi-line expression to always create a new dummy target because m_exe_ctx gets cleared when DoExecute exits. A new input reader has been pushed to handle the input for the expression, which will get popped off and then it was checking the target in m_exe_ctx (which was cleared).

llvm-svn: 173596