The previous analysis was top-down, and worked on a single file (expanding macros). The new analysis is bottom-up, starting from the diagnostics and climbing up the syntax and module tree.
While this is more efficient (and in fact, efficiency was the motivating reason to work on this), unfortunately the code was already fast enough. But luckily, it also fixes a correctness problem: outline parent modules' attributes were not respected for the previous analysis. Case lints specifically did their own analysis to accommodate that, but it was limited to only them. The new analysis works on all kinds of lints, present and future.
It was basically impossible to fix the old analysis without rewriting it because navigating the module hierarchy must come bottom-up, and if we already have a bottom-up analysis (including syntax analysis because modules can be nested in other syntax elements, including macros), it makes sense to use only this kind of analysis.
Few other bugs (not fundamental ti the previous analysis) are also fixed, e.g. overwriting of lint levels (i.e. `#[allow(lint)] mod foo { #[warn(lint)] mod bar; }`.
