Building with dxr-clang

As I'm building yet another program from scratch (I've now run dxr on four separate trees in the past week, and I'm still looking for a good one to use for tests), let me take the time to explain how to set up DXR on your own computer. Instead of stuffing it in this blogpost, most of the information is instead on wiki.mozilla.org. One thing not on that page is that I currently have the sql files in the clang dumper dump to the source directory instead of the object directory, so the DXR config needs to have the source and object directory be the same setting, even if you don't actually build the source in-place!