Auto merge of #123012 - maurer:cfi-supertraits, r=compiler-errors

CFI: Support calling methods on supertraits

Automatically adjust `Virtual` calls to supertrait functions to use the supertrait's trait object type as the receiver rather than the child trait.

cc `@compiler-errors` - this is the next usage of `trait_object_ty` I intend to have, so I thought it might be relevant while reviewing the existing one.

Author: bors