Auto merge of #43886 - oli-obk:clippy, r=nrc
Add clippy as a submodule
~~This builds clippy as part of `./x.py build` (locally and in CI).~~
This allows building clippy with `./x.py build src/tools/clippy`
~~Needs https://github.com/nrc/dev-tools-team/issues/18#issuecomment-322456461 to be resolved before it can be merged.~~ Contributers can simply open a PR to clippy and point the submodule at the `pull/$pr_number/head` branch.
This does **not** build clippy or test the clippy test suite at all as per https://github.com/nrc/dev-tools-team/issues/18#issuecomment-321411418
r? @nrc
cc @Manishearth @llogiq @mcarton @alexcrichton