`Self` in where clauses may not be object safe
Needs crater, virtually certain to cause regressions.
In #50781 it was discovered that our object safety rules are not sound because we allow `Self` in where clauses without restrain. This PR is a direct fix to the rules so that we disallow methods with unsound where clauses.
This currently uses hard error to measure impact, but we will want to downgrade it to a future compat error.
Part of #50781.
r? @nikomatsakis