Auto merge of rust-lang#104889 - GuillaumeGomez:fix-impl-block-in-const-expr, r=notriddle

Fix impl block in const expr

Fixes rust-lang#83026.

The problem was that we didn't visit block expressions. Considering how big the [walk_expr](https://doc.rust-lang.org/nightly/nightly-rustc/src/rustc_hir/intravisit.rs.html#678) function is, I decided to instead implement the `hir` visitor on the struct. It also answers the question which was in a comment for `RustdocVisitor`: we should have used a visitor instead of our ad-hoc implementation.

Adding this visitor also added some extra checks that weren't present before (check changes in `rustdoc-ui` tests).

r? `@notriddle`

Author: bors