Auto merge of #146271 - niacdoial:improperctypes-refactor1, r=tgross35

lint ImproperCTypes: refactor linting architecture (part 1)

This is the first PR in an effort to split  rust-lang/rust#134697 into individually-mergeable parts.

This one focuses on properly packaging the lint and its tests, as well as properly separate the "linting" and "type-checking" code.

There is exactly one user-visible change: the safety of `Option<Box<FFISafePointee>>` is now the same in `extern` blocks and function definitions: it is safe.

r? `@tgross35` because you are already looking at the original

Author: bors