18fa4342fc
Optimize `impl Hash for ObligationCauseData` by not hashing `ObligationCauseCode` variant fields
Split out from #90913 since it's a [clear performance win](https://perf.rust-lang.org/compare.html?start=ad442399756573dccacb314b6bf8079964bcc72a&end=223f5e877fe93b5f437c2d703f883797913cd2b7) and should be easier to review.
It speeds up hashing for `Obligation` [deduplication](c9c4b5d727/compiler/rustc_trait_selection/src/traits/select/mod.rs (L2355-L2356)) by only hashing the discriminant of the `ObligationCauseCode` enum instead of its contents. That shouldn't affect hash quality much since there are several other fields in `Obligation` which should be unique enough, especially the predicate itself which is hashed as an interned pointer.