prepare_bindings.py: enable static bindings

Added a new flag, --allow-static-binding.  When specified,
if (and only if) the swig binary cannot be found, then the
LLDBWrapPython.cpp and lldb.py from the
scripts/Python/{static-binding-dir} are copied into the place where
swig would have generated them.

{static-binding-dir} defaults to static-binding, and can be
overridden with the --static-binding-dir command line argument.

The static bindings checked in are from r253424.

llvm-svn: 253448