Rollup merge of #120870 - Zalathar:allow-min-spec, r=oli-obk

Allow restricted trait impls under `#[allow_internal_unstable(min_specialization)]`

This is a follow-up to #119963 and a companion to #120866, though it can land independently from the latter.

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We have several compiler crates that only enable `#[feature(min_specialization)]` because it is required by their expansions of `newtype_index!`, in order to implement traits marked with `#[rustc_specialization_trait]`.

This PR allows those traits to be implemented internally by macros with `#[allow_internal_unstable(min_specialization)]`, without needing specialization to be enabled in the enclosing crate.