Move code in `rustc_driver` out to a new `rustc_driver_impl` crate to allow pipelining
That adds a `rustc_shared` library which contains all the rustc library crates in a single dylib. It takes over this role from `rustc_driver`. This is done so that `rustc_driver` can be compiled in parallel with other crates. `rustc_shared` is intentionally left empty so it only does linking.
An alternative could be to move the code currently in `rustc_driver` into a new crate to avoid changing the name of the distributed library.
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