Rollup merge of #122416 - Zalathar:levels, r=petrochenkov

Various style improvements to `rustc_lint::levels`

While reading this file, I noticed a few opportunities to make things a little nicer:

- Replace some nested if-let with let-chains
- Tweak a match pattern to allow shorthand struct syntax
- Fuse an `is_empty` check with getting the last element
- Merge some common code that emits `MalformedAttribute` and continues
- Format `"{tool}::{name}"` in a way that's consistent with other match arms
- Replace if-let-else-panic with let-else
- Use early-exit to flatten a method body

Some of these changes cause indentation churn, so ignoring whitespace is recommended.

Author: matthiaskrgr