fix #113702 emit a proper diagnostic message for unstable lints passed from CLI

[kiscad]

Current output:

$ build/host/stage1/bin/rustc hello.rs -Wunnameable_types
warning: unknown lint: `unnameable_types`
  |
  = note: the `unnameable_types` lint is unstable
  = note: see issue #48054 <https://github.com/rust-lang/rust/issues/48054> for more information
  = help: add `-Zcrate-attr="feature(type_privacy_lints)"` to the command-line options to enable
  = note: `#[warn(unknown_lints)]` on by default

warning: 1 warning emitted

Previously, the feature gate diagnostic message is like below, which is the same as the message for unstable lints from the root module.

= help: add `#![feature(type_privacy_lints)]` to the crate attributes to enable

Fixes #113702

[rustbot]

r? @cjgillot

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[kiscad]

r? @petrochenkov

[petrochenkov]

@bors r+

[bors]

📌 Commit d274417 has been approved by petrochenkov

It is now in the queue for this repository.