nodes in statement position.

Extended #[cfg] folder to allow removal of statements, and
of expressions in optional positions like expression lists and trailing
block expressions.

Extended lint checker to recognize lint levels on expressions and
locals.

[breaking change]

And use VariantData instead of P<VariantData> in Item_ and Variant_

This is a [breaking change].

So that lowering is reproducible

paths, and construct paths for all definitions. Also, stop rewriting
DefIds for closures, and instead just load the closure data from
the original def-id, which may be in another crate.

+ Add helper macro for walking lists (including Options)

Make sure Name, SyntaxContext and Ident are passed by value
Make sure Idents don't serve as keys (or parts of keys) in maps, Ident comparison is not well defined

This enables the Debug trait to work on syntax::ast::Stmt

Closes #28527.

Followup on #28440