rustc: Crawl static initializers for reachability

This ensures that private functions exported through static initializers will
actually end up being public in the object file (so other objects can continue
to reference the function).

Closes #13620
auto merge of #13906 : alexcrichton/rust/issue-13620, r=luqmana

This ensures that private functions exported through static initializers will
actually end up being public in the object file (so other objects can continue
to reference the function).

Closes #13620