Auto merge of rust-lang#102944 - nnethercote:ast-Lit-third-time-lucky…

…, r=petrochenkov

Use `token::Lit` in `ast::ExprKind::Lit`.

Instead of `ast::Lit`.

Literal lowering now happens at two different times. Expression literals are lowered when HIR is crated. Attribute literals are lowered during parsing.

r? `@petrochenkov`