[lldb-mi] Fix prompt which can get inserted in the middle of program output in lldb-mi

Summary: The code added in svn r264332 causes "(lldb) " to be printed in the
middle of program console output. This fix restores the behavior for non-Windows
platforms to before the patch.

Reviewers: ted, zturner, clayborg
Subscribers: amccarth, lldb-commits
Differential Revision: http://reviews.llvm.org/D25137

llvm-svn: 283031