Find SWIG with CMake

SWIG is searched under certain paths within python script. CMake can
detect SWIG with find_package(SWIG). This is used iff user checks
LLDB_ENABLE_PYTHON_SCRIPTS_SWIG_API_GENERATION. If
buildSwigWrapperClasses.py does not receive swigExecutable argument,
then the script will use its current search implementation.