Always use python-based SWIG generation script for CMake.

As a first step in addressing Bug #21921 this patch prefers
the python-based SWIG generation by default rather than the
shell-based SWIG generation any time python is enabled
(e.g. LLDB_DISABLE_PYTHON is 0).

Additionally, this patch changes the default value of
LLDB_DISABLE_PYTHON from 1 to 0 on Windows.

Anyone not using the CMake build is unaffected by this patch.