Lots of work on the clang frontend

Unit tests cover a lot more functionality, but clang still fails the
integration tests (possibly due to discovering more functionality during
autoall than gccxml which triggers clang-unrelated bugs).

This code requires the following patched version of clang to function:

    https://github.com/girving/clang/tree/xdress
    https://github.com/girving/clang/commits/e54e651a4f068