Support clippy-driver --wrapper

[Manishearth]

Currently clippy-driver is a RUSTC_WRAPPER, i.e. it takes the name of the rustc executable as its first parameter.

What we should do is make it so that clippy-driver --wrapper is the RUSTC_WRAPPER, and then migrate cargo-clippy over to this. This opens up using the clippy-driver as a direct rustc drop-in in cargo clippy-preview.

This intermediate step is necessary so that cargo-clippy continues to work while we hack on cargo clippy-preview

cc @yaahallo

[Manishearth]

Oh, this is also really three steps:

• First we should add --wrapper and make cargo-clippy and cargo clippy-preview use it.
• Then, we should make non---wrapper calls not consume the rustc argument
• Then, we should make cargo clippy-preview stop using --wrapper, and not pass in a rustc. This is basically making cargo clippy replace rustc (for in-workspace crates only)

This last thing is its own task and probably should be a separate issue on Cargo when we get to it. It might be a bit more involved.

[Manishearth]

After some discussion we realized we don't need this.