libclang: add install/distribution targets for python

Add installation support for the python bindings for libclang.  Add an
additional CMake configuration variable to enumerate the python versions for
which the bindings should be installed.  This allows for a LLVM/clang
distribution to distribute the python bindings for libclang as part of the
image.  Because the python versions need to be explicitly stated by the user,
the default image remains unchanged.

llvm-svn: 340228