Rollup merge of #134603 - kpreid:pointerlike-err, r=estebank

Explain why a type is not eligible for `impl PointerLike`.

The rules were baffling when I ran in to them trying to add some impls (to `std`, not my own code, as it happens), so I made the compiler explain them to me.

The logic of the successful cases is unchanged, but I did rearrange it to reverse the order of the primitive and `Adt` cases; this makes producing the errors easier. I'm still not very familiar with `rustc` internals, so let me know if there's a better way to do any of this.

This also adds test coverage for which impls are accepted or rejected, which I didn't see any of already.

The PR template tells me I should consider mentioning a tracking issue, but there isn't one for `pointer_like_trait`, so I'll mention `dyn_star`: #102425

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