Revert "Implement Debug for C-like enums with a concatenated string"

[nnethercote]

From #155452.

Fixes #162124.

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  • f91c12f

[clarfonthey]

As mentioned in the libs meeting, I kind of consider the debug macros to be a weird edge case and mostly a libs thing even though they're part of the compiler, but I'm not going to bikeshed that particular issue in general.

That said, since major ecosystem breakage is mostly a non-starter for changes like this, I'm going to say it's okay for me to review this and give it my approval. We do probably still want this, but it's going to have to be done in a way that doesn't break existing crates, whether that's via an edition or some extra work on verifying the result of the macro evaluation.

r=me, pending the merge commit fix + CI passing.

r? clarfonthey