Rollup merge of #124687 - fee1-dead-contrib:private-clauses, r=compiler-errors

Make `Bounds.clauses` private

Construct it through `Bounds::default()`, then consume the clauses via the method `Bounds::clauses()`.

This helps with effects desugaring where `clauses()` is not only the clauses within the `clauses` field.

Author: compiler-errors