Rollup merge of #120144 - petrochenkov:unty, r=davidtwco

privacy: Stabilize lint `unnameable_types`

This is the last piece of ["RFC rust-lang#2145: Type privacy and private-in-public lints"](rust-lang/rust#48054).

Having unstable lints is not very useful because you cannot even dogfood them in the compiler/stdlib in this case (rust-lang/rust#113284).
The worst thing that may happen when a lint is removed are some `removed_lints` warnings, but I haven't heard anyone suggesting removing this specific lint.

This lint is allow-by-default and is supposed to be enabled explicitly.
Some false positives are expected, because sometimes unnameable types are a legitimate pattern.
This lint also have some unnecessary false positives, that can be fixed - see rust-lang/rust#120146 and rust-lang/rust#120149.

Closes rust-lang/rust#48054.

Author: matthiaskrgr