Fix ProvenVia for global where clauses

[compiler-errors]

When we're merging one (or more) global where clauses in the presence of no other candidates, ensure that we return TraitGoalProvenVia::ParamEnv so that rigid projections work correctly. This fixes some tests with feature(trivial_bounds).

[rustbot]

r? @BoxyUwU

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[compiler-errors]

r? lcnr

[lcnr]

While correct, I feel like this complicates the controlflow and is a bit confusing to me. I think I'd prefer to instead add the following the "merge all candidates" success branch

let proven_via = if candidates.iter().all(|c| matches!(c.source, CandidateSource::ParamEnv(_)) { TraitGoalProvenVia::ParamEnv } else { TraitGoalProvenVia::Misc };

with that, r=me