Undefined lints scoped under `rustc` don't result in lint unknown-lints getting emitted (unless `-Zunstable-options` is passed)

While undefined "unscoped" lints lead to the compiler emitting the warn-by-default lint unknown-lints

#![allow(undefined)] //~ WARN unknown lint `undefined`

warning: unknown lint: `undefined`
 --> src/lib.rs:1:10
  |
1 | #![allow(undefined)]
  |          ^^^^^^^^^
  |
  = note: `#[warn(unknown_lints)]` on by default

the same doesn't happen for undefined lints scoped under rustc:

//@ check-pass
#![deny(unknown_lints)]

#![allow(rustc::undefined)]

It would be beneficial if rustc would emit unknown_lints in these cases.

Built-in tools like rustfmt and clippy or tools defined via register_tool are responsible for dealing with undefined lints themselves. However, the tool modules lint scope rustc is owned by rustc and should therefore do the checking.

Start by reproducing the two lint examples in the issue and compare the diagnostics with and without -Zunstable-options. Trace rustc's handling of the rustc lint scope and the unknown-lints diagnostic, then add or update a compiler test demonstrating the expected warning for rustc::undefined. Done means rustc emits unknown-lints for undefined lints scoped under rustc without requiring unstable options.