privacy: Rename "accessibility levels" to "effective visibilities" And a couple of other naming and comment tweaks. Related to https://github.com/rust-lang/rust/issues/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.