privacy: Fix type privacy holes when it doesn't cause too much breakage

[petrochenkov]

A subset of #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 (#146470 (comment)).

Closes #144139

[petrochenkov]

@bors try

[petrochenkov]

TODO: @craterbot check p=1 crates=https://crater-reports.s3.amazonaws.com/pr-146470-3/retry-regressed-list.txt

[rustbot]

r? @fmease

rustbot has assigned @fmease.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: compiler
• compiler expanded to 68 candidates
• Random selection from 16 candidates

[rust-bors]

☀️ Try build successful (CI)
Build commit: fcfb0ff (fcfb0ffc7ed7fc631df1136063939ef648434735, parent: 7ad4e69ad585d8ff214f7b42d01f1959eda08f40)

[petrochenkov]

@craterbot check p=1 crates=https://crater-reports.s3.amazonaws.com/pr-146470-3/retry-regressed-list.txt

[craterbot]

👌 Experiment pr-152543 created and queued.
🤖 Automatically detected try build fcfb0ff
🔍 You can check out the queue and this experiment's details.

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more

[craterbot]

🚧 Experiment pr-152543 is now running

ℹ️ Crater is a tool to run experiments across parts of the Rust ecosystem. Learn more