erase types in the move-path abstract domain

erase types in the move-path abstract domain

Leaving types unerased would lead to 2 types with a different "name"
getting different move-paths, which would cause major brokenness (see
e.g. #42903).

This does not fix any *known* issue, but is required if we want to use
abs_domain with non-erased regions (because the same can easily
have different names). cc @RalfJung.