Auto merge of #98867 - cjgillot:metaloop, r=oli-obk

Refactor metadata emission to avoid visiting HIR

This PR refactors metadata emission to be based on tables and iteration over definitions.

In a first part, this PR moves information from the `EntryKind` enum to tables, until removing the `EntryKind` enum.
In a second part, the iteration scheme is refactored to avoid fetching HIR unless strictly necessary.

r? `@ghost`

Author: bors