Make ast validation able to mutate the ast

[oli-obk]

Mostly opening for discussion

cc @petrochenkov @jdonszelmann

I think this could be useful for having to handle weird cases less later in the compiler. Either by replacing things with error nodes (e.g. ty/expr), or by editing out the invalid elements (const fn -> fn in trait methods or trait impls).

Thoughts?

[petrochenkov]

Either by replacing things with error nodes (e.g. ty/expr), or by editing out the invalid elements (const fn -> fn in trait methods or trait impls).

Can do this in AST lowering, which happens not much later than AST validation?

Historically, the use of mutable AST visitors was minimized to prepare for making AST immutable (or as immutable as possible) and allocating in on arena. Of course nobody ended up having enough resources for this large work.