match lowering: build the `Place` instead of keeping a `PlaceBuilder` around
Outside of `MatchPair::new` we don't construct new places, so we don't need to keep a `PlaceBuilder` around.
A bit annoyingly we have to store an `Option<Place>` even though it's never `None` after simplification, but the alternative would be to re-entangle `MatchPair` construction and simplification and I'd rather not do that.
a2122dc1e3