Better closure requirement propagation. Fixes rust-lang/rust#148289 Fixes rust-lang/rust#104477 Should unblock rust-lang/rust#148187 When propagating closure requirements we: - no longer try to find a post dominiting region for the list of non-local lower bounds of `longer_fr`. - we filter the list of `longer_fr-: shorter_fr+` to only these constraints between external regions which are required regardless. If no such constraint exists, we fall back to propagating all of them. See the [FCP](https://github.com/rust-lang/rust/pull/148329#issuecomment-3581019253) for more information. r? `@lcnr`