The old v2 versions of checkout and upload-artifact github actions now produce errors warning they must be updated. The new versions are v3.

The iwyu package has now fixed it's deps and we no longer need to explicitly install libclang-common-9-dev which doesn't exist any more.

The iwyu build target should ignore fileutil.c, and use neater clang output with noisy "note:" output removed.

Run make iwyu-fix to fix includes for tests/rabinkarp_perf.c.