make const_err a future incompat lint
This is the first step for https://github.com/rust-lang/rust/issues/71800: make const_err a future-incompat lint. I also rewrote the const_err lint description as the old one seemed wrong.
This has the unfortunate side-effect of making const-eval error even more verbose by making the const_err message longer without fixing the redundancy caused by additionally emitting an error on each use site of the constant. We cannot fix that redundancy until const_err is a *hard* error (at that point the error-on-use-site can be turned into a `delay_span_bug!` for uses of monomorphic consts, and into a nicely rendered error for [lazily / post-monomorhization evaluated] associated consts).
~~The one annoying effect of this PR is that `let _x = &(1/(1-1));` now also shows the future-incompat warning, even though of course we will *not* make this a hard error. We'll instead (hopefully) stop promoting it -- see https://github.com/rust-lang/rfcs/pull/3027. The only way I see to avoid the future-incompat warning is to use a different lint for "failure to evaluate promoted".~~
Cc `@rust-lang/wg-const-eval`
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