Fix `transmute_undefined_repr` with single field `#[repr(C)]` structs

Fixes: rust-lang#8417

The description has also been made more precise.

changelog: Fix `transmute_undefined_repr` with single field `#[repr(C)]` structs
changelog: Move `transmute_undefined_repr` back to `correctness`

…2, r=Manishearth

Transmute_undefined_repr to nursery again

This PR reinstates rust-lang#8418, which was reverted in rust-lang#8425 (incorrectly I think).

I don't want to start a revert war over this but I feel very strongly that this lint is not in a state that would be a net benefit to users of clippy. In its current form, making this an enabled-by-default `correctness` lint with authoritative-sounding proclamations of undefined behavior does more harm than the benefit of the true positive cases.

I can file a bunch more examples of false positives but I don't want to give the author of this lint the impression that it is ready to graduate from `nursery` as soon as I've exhausted the amount of time I am willing to spend revising this lint.

Instead I would recommend that the author of the lint try running it on some reputable codebases containing transmutes. Everywhere that the lint triggers please consider critically whether it should be triggering. For cases that you think are true positives, please raise a few of them with the crate authors (in a PR or issue) to better understand their perspective if they think the transmute is correct.

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changelog: Re-remove [`transmute_undefined_repr`] from default set of enabled lints