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

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

This is the only change in the GN build patch series that needs a change to a
file outside of llvm/utils/gn: llvm/test/tools/llvm-config/booleans.test checks
the result of llvm-config --build-system for some reason, so I'm updating the
test to accept "gn" as valid output in addition to "cmake". (The alternative
would be to let the gn build self-identify as cmake, which seems worse.)

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

Differential Revision: https://reviews.llvm.org/D56195

llvm-svn: 350410