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

[Urgau]

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)

[rustbot]

Some changes occurred in compiler/rustc_attr_parsing

cc @jdonszelmann

[jieyouxu]

Compiler changes seem fine