Unimplement fmt::Debug for c_void

[Jules-Bertholet]

Taking a shared reference to c_void is never correct, and this impl should never have been added.

This is obviously a breaking change, so needs crater and FCP.

@rustbot label T-libs-api needs-fcp A-FFI

See also #159986, #159935

[rustbot]

r? @Darksonn

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[Mark-Simulacrum]

@bors try

[rust-bors]

☀️ Try build successful (CI)
Build commit: 82b0460 (82b04601f285a69721a2b7d1db460b48a7edeef4)
Base parent: 26ae60a (26ae60a9eeb20b4935be49d7a931a650fa1d2923)

[theemathas]

@craterbot check

[craterbot]

👌 Experiment pr-160115 created and queued.
🤖 Automatically detected try build 82b0460
⚠️ Try build based on commit 03942bc, but latest commit is 588c33e. Did you forget to make a new try build?
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[clarfonthey]

Worth also commenting that there's a blanket Debug impl for *const T and *mut T, and so, this would not affect anything reasonable that uses c_void from deriving Debug.

(Regardless, it would be unsound to actually access data inside these pointers without verifying it's valid, but I could imagine some scenario where someone thought those only implemented Debug when T: Debug, for some weird reason. Those weird reasons don't exist.)