Rollup merge of #109447 - lcnr:coherence, r=compiler-errors

new solver cleanup + implement coherence

the cleanup:
- change `Certainty::unify_and` to consider ambig + overflow to be ambig
- rename `trait_candidate_should_be_dropped_in_favor_of` to `candidate_should_be_dropped_in_favor_of`
- remove outdated fixme

For coherence I mostly just add an ambiguous candidate if the current trait ref is unknowable. I am doing the same for reservation impl where I also just add an ambiguous candidate.

Author: matthiaskrgr