Rollup merge of #109724 - lcnr:prioritize-env, r=compiler-errors

prioritize param env candidates if they don't guide type inference

intended to supersede #109579. We disable the prioritization during coherence to maintain completeness.

Long term we can hopefully replace this hack with adding OR to external constraints at which point the only relevant part when merging responses is whether they guide type inference in the same way.

Reuses `try_merge_responses` for assembly and the cleanest way to impl that was to actually split that so that `try_merge_responses` returns `None` if we fail to merge them and then add `flounder` which is used afterwards which is allowed to lower the certainty of our responses.

If, in the future, we add the ability to merge candidates `YES: ?0 = Vec<u32>` and `YES: ?0 = Vec<i32>` to `AMBIG: ?0 = Vec<?1>`, this should happen in `flounder`.

r? `@compiler-errors` `@BoxyUwU`

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