Rollup merge of rust-lang#105408 - cuviper:help-rpitirpit, r=compiler…

…-errors

Add help for `#![feature(impl_trait_in_fn_trait_return)]`

This adds a new variant `ImplTraitContext::FeatureGated`, so we can
generalize the help for `return_position_impl_trait_in_trait` to also
work for `impl_trait_in_fn_trait_return`.

cc rust-lang#99697