Tell the compiler to tell us that `deriving` is dead.

I'm beginning to suspect it's impossible to avoid accidentally writing
`#[deriving]` at least once in every program, and it results in
non-intuitive error messages: "Foo doesn't have any method in scope
`clone`" despite there being a `#[deriv...(Clone)]` attribute!

Also, lots of documentation around the internet uses `#[deriving]` so
providing this guidance is very helpful (lots of people ask in #rust
about this error).

Fixes #21166.

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