Auto merge of #127404 - compiler-errors:rpitit-entailment-false-positive, r=oli-obk

Don't try to label `ObligationCauseCode::CompareImplItem` for an RPITIT, since it has no name

The old (current) trait solver has a limitation that when a where clause in param-env must be normalized using the same where clause, then we get spurious errors in `normalize_param_env_or_error`. I don't think there's an issue tracking it, but it's the root cause for many of the "fixed-by-next-solver" labeled issues.

Specifically, these errors may occur when checking predicate entailment of the GAT that comes out of desugaring RPITITs. Since we use `ObligationCauseCode::CompareImplItem` for these predicates, we try calling `item_name` on an RPITIT which fails, since the RPITIT has no name.

We simply suppress this logic when we're reporting a predicate entailment error for an RPITIT. RPITITs should never have predicate entailment errors, *by construction*, but they may due to this bug in the old solver.

Addresses the ICE in #127331, though doesn't fix the underlying issue (which is fundamental to the old solver).

r? types

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