Auto merge of #109588 - Nilstrieb:dropless-expr, r=compiler-errors

Alloc `hir::Lit` in an arena to remove the destructor from `Expr`

This allows allocating `Expr`s into a dropless arena, which is useful for using length prefixed thing slices in HIR, since these can only be allocated in the dropless arena and not in a typed arena.

Author: bors