rustdoc: Remove `Foreign{Function,Static}` item types

This series of commits deals with broken links to the source code. It also refactors some repetitive codes from Rustdoc. The most important commit, 1cb1f00d40f000ac7633b62a603db4fcea835ca6, describes the rationale; this will fix a half of #16289.

### Notes on the broken source links

As of bda97e8557820cc4ec13645dbdf976e5ccaa0ce1 (I've used this to check this working as intended), there are 281 (!) such broken links. They can be further classified as follows:

* 178 links to incorrect item types. This is the first half of #16289, and this PR fixes all of them.
* 89 links to redirect pages. They are not technically "broken" but still doesn't give a source code. I have a fix for this in mind, which would make a redirect page slightly *fat*.
* 14 links to incorrect `DefId` in the `gotosrc` parameter. This is #15309, and affects many `liballoc` reexports in `libstd` but *nothing else* (curiously). I'm yet to track this down.
* 0 links to the crate reexported as a different name. This is the second half of #16289, and seems not hard to fix but I'm running out of time.

Prevalence of this kind of bugs calls for a full link verifier integrated into the testing process. :S