Fix double stdout/stderr output from CLI commands in MI mode (MI)

Summary:
This patch fixes stdout/stderr output that printed twice for CLI commands:
was:
```
(gdb)
target create ~/p/hello
Current executable set to '~/p/hello' (x86_64).
Current executable set to '~/p/hello' (x86_64).
^done
(gdb)
```
now:
```
(gdb)
target create ~/p/hello
Current executable set to '~/p/hello' (x86_64).
^done
(gdb)
```

Test Plan: ./dotest.py -v --executable $BUILDDIR/bin/lldb tools/lldb-mi/

Reviewers: abidh

Reviewed By: abidh

Subscribers: lldb-commits, abidh

Differential Revision: http://reviews.llvm.org/D9277

llvm-svn: 235857