The clang frontend passes its first unit test

Unfortunately, none of the integration tests pass now due to
int32 to intc changes.

Due to deficiencies in the clang C and Python bindings, xdress
required a modified version of libclang:

    https://github.com/girving/clang/tree/xdress

In order to make xdress usable with stock clang installs (at least
those with clang 3.3 or later), we may want to factor our modified
version out into a separate repository.