Make ast `MutVisitor` have the same method name and style as `Visitor`

It doesn't map 100% because some `MutVisitor` methods can filter or even expand to multiple items, but consistency seems nicer.

The last commit showcases how similar they are by changing ast validation to a `MutVisitor` (without actually doing any mutation yet). My plan is to replace all nodes that support it with error nodes if validation failed on them. This allows ast lowering to just assume things are error or valid, and avoids having to redo some checks, delaying bugs or checking the global error counter.

tracking issue: rust-lang#127615

…ions, r=petrochenkov

Various ast validation simplifications

Changes pulled out of rust-lang#127524

These are needed to make ast validation a mutable visitor, as we can't keep immutable references to the AST around in that case. But I think they are simplifying things in general and can stand on their own

Make ast `MutVisitor` have the same method name and style as `Visitor`

It doesn't map 100% because some `MutVisitor` methods can filter or even expand to multiple items, but consistency seems nicer.

tracking issue: rust-lang#127615

…nkov

Make ast `MutVisitor` have the same method name and style as `Visitor`

It doesn't map 100% because some `MutVisitor` methods can filter or even expand to multiple items, but consistency seems nicer.

tracking issue: rust-lang#127615