Make LLVM repository configurable.

I used to have issues connecting to the somewhat flaky github repository over at http://llvm.org.git. Timeouts occurred on *every* clone for one more repositories. There exists another mirror at https://github.com/llvm-mirror, which one can now choose explicitly by specifying on the command line, like this:

    ./install-clang -r 'git://github.com/llvm-mirror' <remaining args>

We could certainly hardcode the other mirror and offer a simple flag without argument, but I thought this is the most flexible version.