Rollup merge of #80274 - pierwill:lintlevelsource, r=petrochenkov

Rollup merge of #80274 - pierwill:lintlevelsource, r=petrochenkov

Rename rustc_middle::lint::LintSource

Rename [`rustc_middle::lint::LintSource`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/lint/enum.LintSource.html) to `rustc_middle::lint::LintLevelSource`.

This enum represents the source of a *lint level*, not a lint. This should improve code readability.

Update: Also documents `rustc_middle::lint::LevelSource` to clarify.