Auto merge of #689 - fitzgen:Kowasaki-master, r=fitzgen

Emit `PhantomData<UnsafeCell<T>>` members for all generic parameters

This makes generated generic structs lifetime invariant, since we cannot know
the C++ type's true variance.

Fixes #506

Rebased + squashed version of #635; cc @Kowasaki

To resolve the conflicts and rebase and squash, I did essentially these commands:

```
$ git fetch servo
$ git checkout -b Kowasaki-master servo/master
$ git merge --squash --edit Kowasaki/master
$ cargo test
$ cd tests/expectations
$ cargo test
$ cd -
$ git commit --author="Kowasaki"
$ git push fitzgen Kowasaki-master
```

Hopefully that is useful for you in the future :) Thanks for fixing this issue!

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