Rollup merge of #140894 - Urgau:check-cfg-rustdoc, r=GuillaumeGomez

Make check-cfg diagnostics work in `#[doc(cfg(..))]`

This PR makes it so that the check-cfg `unexpected_cfgs` lint, is correctly emitted in `rustdoc`'s `#[doc(cfg(..))]`.

This is achieved by adding a custom trait to `cfg_matches` (the method that emits the lint) which permits `rustc` and `rustdoc` to each have their way to emitting lints (via buffered lints/AST for `rustc` and via `TyCtxt`/HIR for `rustdoc`).

The reason this is required is because buffered lints operates on the AST but `rustdoc` uses the HIR and by the time `rustdoc` calls `cfg_matches` we are way passed the point where buffered lints have been drain and emitted.

Best reviewed commit by commit.

r? `@jieyouxu` (for the compiler part)
r? `@GuillaumeGomez` (for the rustdoc part)

Author: tgross35