Rollup merge of rust-lang#103653 - GuillaumeGomez:missing-impl-private-json, r=notriddle

Add missing impl blocks for item reexported from private mod in JSON output

Fixes rust-lang#102583.

Since we don't inline for the JSON output, the impl blocks from private modules are not present when we generate the output. To go around this limitation, in case the impl block doesn't have `#[doc(hidden)]` and is implementing a public item, we don't strip it.

cc `@fmease` `@aDotInTheVoid`
r? `@notriddle`

Author: GuillaumeGomez