Make generic const type mismatches not hide trait impls from the trait solver
pulled out of https://github.com/rust-lang/rust/pull/119895
It does improve diagnostics somewhat, but also causes some extraneous diagnostics in potentially misleading order.
The issue was that a const type mismatch, instead of reporting an error, would silently poison the constant, only for that information to be thrown away and the impl to be treated as "not matching". In #119895 this would cause ICEs as well as errors on impls stating that the impl needs to exist for itself to be valid.
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