core: avoid `extern type`s in formatting infrastructure
```@RalfJung``` [said](https://rust-lang.zulipchat.com/#narrow/stream/219381-t-libs/topic/Use.20of.20.60extern.20type.60.20in.20formatting.20machinery/near/446552837):
>How attached are y'all to using `extern type` in the formatting machinery?
Seems like this was introduced a [long time ago](34ef8f5441). However, it's also [not really compatible with Stacked Borrows](https://github.com/rust-lang/unsafe-code-guidelines/issues/256), and only works currently because we effectively treat references-to-extern-type almost like raw pointers in Stacked Borrows -- which of course is unsound, it's not how LLVM works. I was planning to make Miri emit a warning when this happens to avoid cases like [this](https://github.com/rust-lang/rust/issues/126814#issuecomment-2183816373) where people use extern type specifically to silence Miri without realizing what happens. but with the formatting machinery using extern type, this warning would just show up everywhere...
>
> The "proper" way to do this in Stacked Borrows is to use raw pointers (or `NonNull`).
This PR does just that.
r? ```@RalfJung```
The tests in this directory are shared by two different test modes, and can be
run in multiple different ways:
./x.py test coverage-map (compiles to LLVM IR and checks coverage mappings)
./x.py test coverage-run (runs a test binary and checks its coverage report)
./x.py test coverage (runs both coverage-map and coverage-run)
Maintenance note
These tests can be sensitive to small changes in MIR spans or MIR control flow,
especially in HIR-to-MIR lowering or MIR optimizations.
If you haven't touched the coverage code directly, and the tests still pass in
coverage-run mode, then it should usually be OK to just re-bless the mappings
as necessary with ./x.py test coverage-map --bless, without worrying too much
about the exact changes.
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