Auto merge of #124952 - compiler-errors:no-error, r=lcnr

Rename some `FulfillmentErrorCode`/`ObligationCauseCode` variants to be less redundant

1. Rename some `FulfillmentErrorCode` variants.
2. Always use `ObligationCauseCode::` to prefix a code, rather than using a glob import and naming them through `traits::`.
3. Rename some `ObligationCauseCode` variants -- I wasn't particularly thorough with thinking of a new names for these, so could workshop them if necessary.
4. Misc stuff from renaming.

r? lcnr

compiler/rustc_const_eval/src/transform/check_consts/check.rs

@@ -738,7 +738,7 @@ impl<'tcx> Visitor<'tcx> for Checker<'_, 'tcx> {
                 let cause = ObligationCause::new(
                     terminator.source_info.span,
                     self.body.source.def_id().expect_local(),
-                    ObligationCauseCode::ItemObligation(callee),
+                    ObligationCauseCode::WhereClause(callee),
                 );
                 let normalized_predicates = ocx.normalize(&cause, param_env, predicates);
                 ocx.register_obligations(traits::predicates_for_generics(