[eddyb]

We explicitly support building against librustc_driver-*.so for both "custom drivers" (what we call those binaries I mentioned) and generally "rustc as a library" usecases. We should maybe rename it to librustc and remove as much potentially-confusing "driver" terminology as possible.

Pinning a nightly and installing the "rustc-dev" rustup component are both necessary, the former because internal APIs of rustc aren't stable, but it's a supported usecase.

Both clippy and miri are developed out-of-tree like that, and sync'd using `git subtree` (or `git submodule` but we want to move everything to subtree).