Annotate dead-code lint with a note that Debug impls are ignored

[nikomatsakis]

PR #85200 modified the dead-code lint so that it ignores Debug impls (and perhaps a few others, I forget) for the purposes of dead-code detection. This is pretty reasonable, but it does create false dead-code warnings when one has structs that exist solely to be debug printed. As evidenced by #88900, users are confused by this in practice.

@pmetzger had a good suggestion:

Even if this warning is reasonable, it would be a good idea if the message explained that derive(Debug) is ignored for purposes of the lint. As it happens, I spent a lot of time tearing my hair out trying to figure out why the warning was being triggered. (In my case I have structs that exist only to be dumped to a log using debug prints.)

I think the ideal would be to note that an "ignored impl" is present and issue a note on the lint. I'm not sure how easy that would be to do!

[nikomatsakis]

cc @FabianWolff, author of #85200 -- would you be interested in taking a look at this?