Types used only in `extern` blocks seem to be dead

[emberian]

compile librustdoc/lib.rs for example. it marks struct buf as dead.

cc @ktt3ja

[ktt3ja]

Hmm, strange. I thought I already handled that case. I even have a test case for it:
https://github.com/mozilla/rust/blob/1b12dca7f97a51c6cbb4f47ea6e095d841a97c1a/src/test/compile-fail/lint-dead-code-3.rs#L45-L50

[huonw]

Shouldn't that test-case actually warn because free is unused and not exported?

[huonw]

Fixed by #10870.