Fix darwin-debug build in cmake (OS X)

This patch fixes test_launch_in_terminal test which doesn't work
in OS X since the moment as it was added in r225284. The test fails
because Target::Launch returns the following error: "the darwin-debug
executable doesn't exist at
<output_dir>/lib/python2.7/site-packages/lldb/darwin-debug'".

Patch by Ilia K
Differential Revision: http://reviews.llvm.org/D7102

llvm-svn: 227096