Fix some more `non_lifetime_binders` stuff with higher-ranked trait bounds
1. When assembling candidates for `for<T> T: Sized`, we can't ICE because the self-type is a bound type.
2. Fix an issue where, when canonicalizing in non-universe preserving mode, we don't actually set the universe for placeholders to the root even though we do the same for region vars.
3. Make `Placeholder("T")` format like `T` in error messages.
Fixes#108180Fixes#108182
r? types
c5d5c57666