Also add a build file for llvm-lit, which in turn needs llvm/tools/llvm-config.
With this, check-lld runs and passes all of lld's lit tests. It doesn't run any of its unit tests yet.
Running just ninja -C out/gn will build all prerequisites needed to run tests, but it won't run the tests (so that the build becomes clean after one build). Running ninja -C out/gn check-lld will build prerequisites if needed and run the tests. The check-lld target never becomes clean and runs tests every time.
llvm-config's build file is a bit gnarly: Everything not needed to run tests is basically stubbed out. Also, to generate LibraryDependencies.inc we shell out to llvm-build at build-time. It would be much nicer to get the library dependencies by using the dependency data the GN build contains (http://llvm-cs.pcc.me.uk/gen/tools/llvm-config/LibraryDependencies.inc#1).