Add compiler-builtins to bootstrap

[tgross35]

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[tgross35]

Bootstrap people, I think I need some help here making this actually work :) ideally these tests should probably run via path (./x t compiler-builtins, ./x t library/compiler-builtins) as well as under ./x library, but I can't quite figure that out.

It would be good for this to run on CI, as well.

[Kobzol]

Left a few comments. I noticed that compiler-builtins has its own workspace, but at the same time, it doesn't have a Cargo.lock file. I wonder how do we want to deal with that, the dependencies probably should be locked. Either we should add a lockfile to it, or we should somehow make it a part of the root workspace? 🤔

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[bors]

☔ The latest upstream changes (presumably #145663) made this pull request unmergeable. Please resolve the merge conflicts.