Auto merge of #110243 - WaffleLapkin:bless_tagged_pointers🙏, r=Nilstrieb

Tagged pointers, now with strict provenance!

This is a big refactor of tagged pointers in rustc, with three main goals:
1. Porting the code to the strict provenance
2. Cleanup the code
3. Document the code (and safety invariants) better

This PR has grown quite a bit (almost a complete rewrite at this point...), so I'm not sure what's the best way to review this, but reviewing commit-by-commit should be fine.

r? `@Nilstrieb`

Author: bors