rustdoc: (WIP) Allow the doc generation from compiled crates.

This is a work in progress and cannot be merged yet (there are several known issues). I want to gather more opinions about this feature, especially because, I came to believe that this is a lot harder than "putting a few more wires in place" and considerably delicate feature to implement.

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This commit uses an existing foreign inlining facility to implement the doc generation from compiled crates. At the highest level, this is a *huge* hack since the metadata doesn't provide everything we need to generate docs.

Fixes #2206. Will fix #15309 once integrated with Makefile.