Rollup merge of #99383 - ouz-a:issue_57961, r=oli-obk

Formalize defining_use_anchor

This tackles issue #57961

Introduces new enum called `DefiningAnchor` that replaces `Option<LocalDefId>` of `defining_use_anchor`. Now every use of it is explicit and exhaustively matched, catching errors like one in the linked issue. This is not a perfect fix but it's a step in the right direction.

r? `@oli-obk`

diff --cc compiler/rustc_infer/src/infer/opaque_types.rs
Simple merge