fix: use `BoundVar`s from current generic scope
Fixup for #13335, addresses https://github.com/rust-lang/rust-analyzer/pull/13339#issuecomment-1266654607
Before the change in generic parameter order, `BoundVar`s for trait reference didn't change whether you are in an impl's scope or in an associated item's scope. Now that item's generic params come before its parent's, we need to shift their indices when we are in an associated item's scope.
476d043874