Rollup merge of #121813 - Urgau:misc-non_local_defs-lint, r=cjgillot

Misc improvements to non local defs lint implementation

This PR is a collection of small improvements I found when I [needlessly tried](https://www.github.com/rust-lang/rust/pull/120393#issuecomment-1971787475) to fix a "perf-regression" in the lint implementation.

I recommend looking at each commit individually.

Author: matthiaskrgr