Add linting for crate_type attribute values.

[am0d]

This ensures that the crate_type attribute always contains a value,
and does not contain an invalid value.

Fixes #11256.

[alexcrichton]

I'd like to avoid duplicating valid crate_type values across the compiler, so I think this should use the add_lint api from here: https://github.com/mozilla/rust/blob/master/src/librustc/driver/session.rs#L429 instead

[am0d]

Are you suggesting that collect_outputs should be the one to call add_lint if the crate_type doesn't match?
I can move the code there if so.

[alexcrichton]

Indeed, the None case means that it wasn't of the form crate_type = "value" and the Some(s) fall through case means that it wasn't recognized.

[am0d]

Ok. The only thing I can't work out is how to get a reference to a Session object in collect_outputs so that I can call the add_lint method on it. The options parameter doesn't have any reference to the session, and collect_outputs is not a method on Sesssion.

[alexcrichton]

I would recommend adding a parameter of type Session.

[am0d]

Ok. There are only a couple of spots that collect_outputs is called, so I will try that.

[am0d]

@alexcrichton I've re-written this PR with your suggestions. This also now adds a new lint type unknown-crate-type which is set to deny by default.