Change SWIG output directory when building LLDB.framework with CMake

Instead of assuming that SWIG generated files (e.g. lldb.py) will live
in scripts, we should set it to $LLDB_PYTHON_TARGET_DIR. This variable is set to
scripts, except when building LLDB.framework when it is set to
LLDB_FRAMEWORK_DIR.

Differential Revision: https://reviews.llvm.org/D47742

llvm-svn: 333968