Auto merge of rust-lang#102233 - petrochenkov:effvis, r=jackh726

privacy: Rename "accessibility levels" to "effective visibilities"

And a couple of other naming and comment tweaks.

Related to rust-lang#48054

For `enum Level` I initially used naming `enum EffectiveVisibilityLevel`, but it was too long and inconvenient because it's used pretty often.
So I shortened it to just `Level`, if it needs to be used from some context where this name would be ambiguous, then it can be imported with renaming like `use rustc_middle::privacy::Level as EffVisLevel` or something.

Author: bors