Rollup merge of #152543 - petrochenkov:rprivtit2, r=fmease

privacy: Fix type privacy holes in RPITITs

A subset of rust-lang/rust#146470.

Private types in RPITITs now report hard errors.
Private types in bounds of associated types still only report the `private_bounds` lint due to unacceptable amount of breakage (rust-lang/rust#146470 (comment)).

Closes rust-lang/rust#144139

Author: JonathanBrouwer