Rollup merge of #115744 - fmease:fix-e0401, r=compiler-errors

Improve diagnostic for generic params from outer items (E0401)

Generalize the wording of E0401 to talk about *outer items* instead of *outer functions* since the current phrasing is outdated. The outer item can be a function, constant, trait, ADT or impl block (see the new UI test for the more exotic examples).

Further, don't suggest introducing generic parameters to constant items unless the feature `generic_const_items` is enabled.

Lastly, make E0401 translatable while we're at it.

Fixes #115720.

Author: matthiaskrgr