Add scary warnings to errors-downgraded-to-warnings in borrowck=migrate.

Add scary warnings to errors-downgraded-to-warnings in borrowck=migrate.

Also convert an ICE that became reachable code under borrowck=migrate
into a normally reported error (which is then downgraded to a
warning). This actually has a nice side benefit of providing a
somewhat more useful error message, at least in the particular case of
the example from issue #27282.