we should test LLVM integration

[regehr]

We should have testing for alive-tv and the tv opt plugin. This isn't too hard, @regehr will work on it at some point. We should also have CI do this, which requires CI to have access to an LLVM compiled with RTTI/EH, which seems to mean that we have to build our own LLVM. This isn't too difficult via docker, and I have this working, but what I haven't done is figured out how to integrate tests that run in a custom docker image with the other tests in our .travis.yml.

[regehr]

docker does not support OS X images, as far as I can tell, so we'll only test LLVM integration on Linux. this shouldn't be too big of a problem.