[gn build] Add check-clang target and make it work

With this, check-clang runs and passes all of clang's lit tests. It doesn't run
any of its unit tests yet.

Like with check-lld, 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-clang will build
prerequisites if needed and run the tests. The check-clang target never becomes
clean and runs tests every time.

Differential Revision: https://reviews.llvm.org/D56095

llvm-svn: 350108