Rollup merge of rust-lang#139235 - nnethercote:AstValidator-tweaks, r=compiler-errors

`AstValidator` tweaks

When I read through `AstValidator` there were several things that tripped me up, and made the code harder to understand than I would have liked. This PR addresses them. Best reviewed one commit at a time.

r? `@davidtwco`

Author: matthiaskrgr