Auto merge of #101386 - aDotInTheVoid:rdj-discriminant, r=GuillaumeGomez

Auto merge of #101386 - aDotInTheVoid:rdj-discriminant, r=GuillaumeGomez

Rustdoc-Json: Add enum discriminant

Does the first part of #101337, by adding it to `clean`, but doesn't change HTML output, as

1. [No Consensus has appeared on the UI for this](https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/Enum.20discriminant.20values.20in.20HTML.20output)
2. [When inlining across crates, information is lost](https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/.60clean_variant_def.20.60vs.20.60clean_variant_data.60)

JSON doesn't have either of these limitations.

r? `@GuillaumeGomez`