Auto merge of #50966 - leodasvacas:self-in-where-clauses-is-not-object-safe, r=nikoma...

Auto merge of #50966 - leodasvacas:self-in-where-clauses-is-not-object-safe, r=nikomatsakis

`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