cmake fixes for lldb target.

ninja lldb now does the following:
* forces the python post-build step to fire, which sets up the python lldb module properly.
* on Darwin and Linux, requires the lldb-server target to be built.
* on Darwin, requires the debugserver target to be built.

See http://reviews.llvm.org/D12899 for details.

llvm-svn: 247810
3 files changed