Implement PhantomData

[bjorn3]

This is the full definition:

#[lang = "phantom_data"]
pub struct PhantomData<T: ?Sized>;

PhantomData is a ZST, but in all other aspects it behaves as if it contains a value of type T. For example wrt to variance. It also counts as a use of a generic parameter and thus allows disabling the error that generic parameters need to be used.

[liushuyu]

I think this issue could be tagged good-first-pr since a naive implementation (which does not actually check whether the annotated type is really a ZST) is pretty straightforward.

Hints for new contributors: Check this pull request: #1100.

[tamaroning]

Hi, I want to take a shot at this issue.